that "make the front hub" disappear thing is a decent rule of thumb, but ultimately you're going to have to figure out what's comfortable.

if you're asking if a 90 will suffice because you have one on hand, try it and see if you feel cramped or if you feel okay. we can't tell you what's going to be comfortable.

some 25.4 stems can be spread to accept 26 bars, but i prefer not doing that. stems that have removable faceplates can accommodate a decent range of bar sizes. most of my bikes are threadless.