Hm... Do you mean to allow the merger to specify the value of the commit? E. g. by default it could be 1%, but merger can make it smaller if commit is not very valuable. Sounds like a good idea, thanks!
One plausible possibility that might help: in the block that normally contains Signed-off-by and Reviewed-by, add a tag for marking a commit as "minor" or "trivial", in the sense used by the Linux kernel's trivial@ patches or the GNU project's threshold for changes accepted without copyright assignment.
And as a quick hack, scale the tip by log(diffstat) with a sensible upper bound.