Clamp it so it lines up with the bottom flush with the cap on the angled knee wall and then scribe your lines… I install handrails. That is how you do it. (most quickly and accurately anyway). Then you cut one side. Move your clamps up to where your handrail is going to be placed and check to see if the other line you haven’t cut yet is where it needs to be…(your wall and that post might not be plumb or there might be some wowing going on).

Eyeball it to make sure the second cut is where it should be. The angle should be the same as your first cut (maybe a bit more or less but if everything is plumb then it will be the same). Then put your piece of wood up and mark for spindles… if there is a slight (like really small) gap between your cuts and the wall and post, when you attach it it should suck in and get pretty tight.. just use some wood filler if there are noticeable gaps.

I’m guessing both the post and wall have good attachment points (solid framing timber beneath drywall) if not then attach it to whatever has the most solid attachment point first and the weakest last.