Hiya! Glad to see you back at it! In my armors the half circles are 5cm and 5.5cm at their widest and 2.5cm and 2cm at their tallest. The half circles are not deep at all, just a slight indent. But not all armors are the same size. The important thing is that it be proportionate to yours.
There are some details missing in your back armor, mainly in the greeblies, but the crl doesn’t make specific mention of them, so I wouldn’t be too concerned. I think your main edit is going to be that half circle.