After a few weeks busy with other things, I finally got a few hours to see if I can dial in perfect 90.
Still not having much luck, not sure what I'm doing wrong, so I'll seek an alternative to a perfect shooting board.
I'll try my hand at planing to a scribed line or maybe even the Paul Sellers vice-held end grain shooting jig.
Since I'm only making stuff for myself & family, I've decided perfection can wait - I'd much rather spend the few hours I get woodworking making something, even if imperfect, than getting frustrated over one part of the process. I'm sure I'll figure out what's wrong eventually