I can try and give you a condensed description of a method I use that will let you put markers basically anywhere you need easily. It's a extended version of the 'T' Shortcut that you should have if you're using the C3 stuff. Basically, there is a Q and a T shortcut. The Q is the default, which maps based on the exiting Time Signature (usually 4/4). T, on the other hand, will let you adjust the signature if needed.
First you need to Create the Macro. While on the main screen of Reaper, go to Actions -> Show Action List, and then select new. Name it whatever (mine is Tempo by Measure). As for the macro itself, uses these commands in order:
- Time selection: Set start point
- Markers: Go to next marker/project end
- Time selection: Set end point
- Markers: Create measure from time selection (new time signature)...
Once finished, set the macro to something you will remember; I use shift+Q so it matches with the existing Q. There is a second thing you need to do for this to consistently work. File -> Project Settings -> change Timebase for items/envelopes/markers to "Time"
From there, do the usual steps to start a new tempo map in a project. Get the initial tempo and time signature, and then make sure your 3.1 lines up (or whatever you need it to fall on).
From here, we can start using the new macro. To make your life much, much easier as you'll see later, you're going to watch to take whatever your time signature is at the beginning, and make it reduced down to 1/x (ie, 4/4 becomes 1/4, 3/4 becomes 1/4, 7/8 becomes 1/8). In essence, this will make every "beat" a measure to Reaper at this time. You can double check by seeing that the start of the song now falls on 9.1.00 instead of 3.1.00.
How this macro actually works is technically "backward" from how Q and T work by default. Instead of going from the cursor back to the previous measure and creating from there, it actually goes from the cursor FORWARD, to a marker that you place down (in case you don't know, you place a marker down by pressing 'M' on the main screen, and it'll drop a marker on your current location). Assuming your project was set up correctly, your first downbeat should be in the correct location. This is where the macro comes into play.
Let's say the song is 4/4, and starts at 3.1 like normal (9.1 in this case since we made the initial time signature 1/4). Starting at 9.1, you can go out four measures (ie, beats) to 13.1 and drop a marker on the audio where the downbeat actually falls (which should be around there). You'll have to turn "snap to grid" off while placing these. Once the marker is down, go back to 9.1 (you may turn snap back on to ensure you are on 9.1 exactly), and then use the macro. It'll pop up an 'Edit Time Signature' box. If you set up everything correctly, the box will be showing:
- Time Signature: 1 / 4
- Bars: 4
- Tempo: (whatever the tempo will be)
You can see what this is doing. It's starting from your cursor location at 9.1, and then going out to your marker around 13.1. Reaper knows you are in a time signature of 1/4, and it sees 4 "measures" (again, beats technically). Once you hit enter, it'll create that measure for you. This is the process. Place a marker a head of yourself, and then use the macro. You can place ALL the markers if you want first, and then go back and spam your macro -> enter through the whole song if you want.
How does this help? Well, because we changed the first time signature to 1/4, Reaper will treat each beat as a measure. By default, you'll be placing markers down every four "measure", but there's nothing stopping you from doing them whenever you wanted. Let's say the chorus to a song is 4/4 except for one measure of 2/4 at the end. Well, once you get to that part, just drop the next marker two "measures" a head instead of four. It'll create a measure for just those two beats.
Another example, which happens to me a lot... Let's say every other measure has nothing of value on the downbeat. The odd measures all have a kick drum, while the even ones don't type thing. Instead of trying to fudge a tempo marker where you can't reliably hear anything, you can use this to map 'around' it. Say there are snares on the 2 and the 4 like most rock songs. Well, you can have a marker on the downbeat, and then go three beats out and drop a marker on that snare. You can then drop another marker two beats out from there (which would be the next measure over that 'silent' downbeat), and then drop a final marker on the following downbeat. When you go through with the macro on that, it'll create measures of 3 bars, 2 bars, and then 3 bars. Which, in the end, adds up to 2 measure of 4/4 as it should.
When using this, you could technically tempo map every beat if you wanted. This is great for songs with a slowdown because you can drop a marker easily when you need to on each beat as it slows. It's also great for songs which have terrible downbeat waveforms to work with. You can tempo map to the first snare, and then just do markers on the snares instead.
The biggest thing, is that once you are finished, you'll want to change back the 1/4 at the beginning. Once you do, you'll see the whole project fall back into place as it should. The only thing you need to remember is to go back and manually add back in any Time Signature changes that might happen. I know that's a lot and it might be hard to understand, but I think there is enough there to work with. You don't HAVE to change the signature to 1/x if you don't want to. This will work THE SAME if you don't. Mainly, because each time you run the macro it'll ask you what you want your TS and Bars to be. You can fudge anything to work with that. Only issue is that if you have to use the alternating 3/4, 2/4, 3/4 issue that I explained, not doing it under 1/4 will mean you'll have to remove ALL those time signatures afterwards.
Here's an example I quickly pointed out from Rush's The Analog Kid. https://imgur.com/a/l1RXcSQ
The song is in 4/4. However, at M48, I noticed that doing one full measure tempo wouldn't line up. Thus, I used markers a 48.3 and 48.4 to subdivide the measure up with different tempos that better match what's being played. You can also see the 3, 2, 3 thing I was talking about on measures 49 and 50. The downbeat of M50 I wasn't confident on, so I mapped the tempos using the snare on beat 4 of M49 and the snare on beat 2 of M50 around it. After M50, it goes back to the 4 beat measures.