"It’s a process of playing a riff, figuring out the comfortable tempo and time signature, setting it in Reaper, then playing along to it until there’s a change

"It’s a process of playing a riff, figuring out the comfortable tempo and time signature, setting it in Reaper, then playing along to it until there’s a change and doing it again." Not sure it helps, but you can use a tempo map in Reaper to basically warp the grid to your audio. In Classical and Romantic music where Tempo Rubato is pretty common this is a life-saver.