The easiest way for you to approximate this would be to get a piece of scrap cut at 37degrees on one end and rest it on the cap wall and slide it up next to the post or wall and then make a scribe.

Almost all stair cases fall in the 36-39 degree range.

Cut your rail long and sneak it in. You’ll have to make adjustment cuts on the post and wall. It’s not uncommon for to have to cut a 37 at the post and a 38 with a 1 degree adjustment at the wall because of how out of plumb/square the framing is.