Forget the ‘planing’ stuff; the wheel and chain will always be parallel-ish to each other regardless of hub width or spindle length. In an ideal world, the measurement to watch is the distnance between the center of the downtube and the outer chain ring. Depending on whether you are running a double or triple crank, there are are standards for this- Sheldon Brown is your friend here.
In my universe, where friction front downtube shifters reign, I simply strive to get the inner-most ring fairly close to the chainstay- within a couple mm. Sometimes this is the result of experimentation using one or more of the handful of square taper BB’s on hand; sometimes it is a lucky coincidence. If using STI indexed front shifters, things need to be more by the book.
It’s not clear whether you are using the same crankset that came on the bike and are just replacing the spondle that was destroyed during removal. If you are, and you know the previous BB was a 122, then just buy another one. The good news is that 7 speed setups are remarkably tolerant of this like this, so it’s a good setup for a first timer and will likely work fine no matter what you do.
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