The way that the distro works is that it looks at the 5 players on the floor in comparison to each other. Lets say your 5 players have this distro:
PG-5, SG-8, SF-11, PF-3, C-7 (total is 34)
The computer computes the distro as a percent of that total, so it would come out as:
PG-15% (5/34), SG-24%, SF-32%, PF-9%, C-21%
This, of course is before the computer takes into account the defense you are facing, any double teams, etc.
Now lets say the SF is substituted out and the back up has a distro of 3. That leaves us with PG5, SG8, SF3, PF3, C7 (total 26). And the new distros would become PG-19% (5/26), SG-31%, SF-12%, PF-12%, C-27%. Notice that the shooting guard's distro went up 7% just because of the substitution.
So when setting the distros, don't worry about the total. Worry about ratios.