It would really be helpful if there was a better way to set compatibility values for Titlebar Buttons.

I noticed (suddenly) my Excel 365 seems to have the buttons way off (all over the account area) so I tried deleting the compatibility rule, then adding one and setting the Offset to 0, 10, 20, -10, -50, 50 and none of them changed the position at all. It finally moved at 100, and stopped around 110 (i.e. 110, 120 and 130 put them in the same position).

It would be nice if the settings overrode everything, and just moved the buttons. It would be even nicer if you could drag them around, or had a button to pop-up rulers where they go showing what sample values would mean.

Agreed! It takes a fair amount of experimentation to figure out what works.