I was going to do an ACE ultralight stock, and when it is mounted at a perfect straight line the guy in that thread said it was uncomfortable and preferred a slight downward slant, and added a shim to achieve that.
Most factory AK's have a slight downward angle to their stocks:
Whereas the ACE ultralight mounted on a flat-backed receiver would look like this:
So, he added an angled shim to match the factory angle:
It's not an AK, but you get the idea. A slight angle may not be a bad thing is all I'm saying.