I've seen the ACP approach work on soft metals but when I tried it on mild steel it wasn't so good, as you say.
Masking tape with guidelines drawn on it sometimes helps with alignment, if only to reduce the tendency for the stamp to skate around, though I'm guessing that won't be a problem on leather.
It sounds like you've tried harder than I have! It's reassuring (for me) to believe there isn't an easy solution and my badly aligned lettering is just how things are.
I've seen the ACP approach work on soft metals but when I tried it on mild steel it wasn't so good, as you say.
Masking tape with guidelines drawn on it sometimes helps with alignment, if only to reduce the tendency for the stamp to skate around, though I'm guessing that won't be a problem on leather.