I've been into leathercrafting since three years now and I still have problems getting angles on the backside sometimes. I've watches the beginners course already just to see if I've a bad habit somewhere but I do it excactly like shown there.
I wonder if the thinkness of the leather has an impact. In all videos explaning the saddle stich a rather think leather is used. If I use think leather or two pieces of leather to stich together I manage the angle on the backside. But if I use thinner leather like in my current project (1,2mm calf leather with 0,5-0,8mm lamb leather lining) I cant manage to get the angle on the backside even with the cast. Any ideas what I'm doing wrong?
In my opinion, you are doing nothing wrong. You have acceptable angles on the back. The back angles will always look less angled than the front ones. That is normal.
I also think you need to practice a bit more your consistency of hand movement and pull. In stitching N1, there are two stitches (no 8 and 9 starting from the right) that indicate that the needles did not pass each other in the same position as the rest, which gives you 2 slightly elevated and flattened stitches.
On the other hand, 3.85mm paired with 0.4mm thread is not optimal. I would try to stitch with at least 0.6mm and i think you will find better results.