Originally Posted by
Bill Kapaun
OP isn't BUILDING A BIKE!
The OP is changing a crank/BB and wants their chain to run as straight as possible for the rings/cogs they use the most.
I do exactly the same thing! I use the longest BB that the FDER will span.
Both my bikes are 3X9 that I use as a 1X9 about 98% the time on the middle ring.
Even then, we're talking about the thickness of about 1.5 cogs.
I cannot count the bikes I've seen where somebody just couldn't seem to get their shifting quite right, no matter what they did. It's fine in the lower gears but something rubs or doesn't shift in the higher gears, or vice versa, or somethings just not quite working perfectly as it was designed to do from the factory.
Assuming no damage (bent derailleur hanger, etc.) first thing to check is chainline and many times someone has installed the wrong bottom bracket. Shimano BB sizes are spaced closely enough that you can always come within a millimeter of spec. But, you have to know what it should be for your bike, and actually measure.
If you start with a good chainline and everything else is also within spec, your bike will shift properly and smoothly as it did when it came from the factory. You won't have to keep it in any certain gear range.