most shops use the following rule of thumb:

25mm max for 1" steerer
35mm max for 1 1/8" steerer

deviate from that at your own risk because it may affect warranty.

my personal view is that if you have a shorter stem, you can use more
spacers, longer, less so. it's the total /leverage/ that's important,
and that's simple trig for the distance from the center at the top of
the bearing and the center of the handlebar [assuming straight bars].

jb