Too small and nobody redeems it. Too large and you are giving away margin you did not need to.
Getting this right is less about clever tooling and more about consistency. Pick a rule, apply it to every customer, and let the results accumulate over a full year before you judge it.
What to try first
Start with a single segment and a single offer. Once the numbers are steady, add a second variant and compare them honestly over at least a few hundred sends.
The programmes that work are almost always the boring ones that simply ran for long enough to matter.