The tempo is baked in to flac file, so you have to match the reaper project to that. It is dealt with a in kenny vid, but there are so many kenny vids now it makes it tough to find the right bit in the right one!
If the track you're playing to is a fixed tempo then my favourite quick way to work it out is actually this:
https://www.all8.com/tools/bpm.htm
There is a JS that does the same thing but I can't remember what it's called.
Tap the tempo, set the project to the correct tempo and then adjust the start point of the flac file so you've got the beats properly lined up to the grid. You'll then be able to use the quantise tools to bring your performance in time with the flac.