Checking your écorché model is absolutely a good thing, and placing muscles will get easier with practice (also ref with visible muscles can help). Keep going :D

I agree with Mathew Patterson, drawing the muscles in details might help you get a better understanding of the arm. I'll add to this: the bony landmark you placed on the flexors side is good, but in this position, you won't see the bony landmark on the extensors side, as the ridge muscles will hide it. You're also going a bit too thin with the wrist just before the hand. A good portion of the forearm looks like a box, as it is mostly bones and tendons. Don't hesitate to draw that box so you maintain the wrist volume and shape, and learn to assess where front plane and side plane stand.