If no one else has started on this, I can give it a shot!

Go for it

Afaik max_notes_per_denomination has to be target_per_denomination denomination_base - 1

If I understand correctly, the issue here is that we want to have enough notes of every denomination to be able to make target_per_denomination number of offline payments, each of potentially any size. For each offline payment, the worst case is that we need a number of notes for a single denomination that is one less than the base, so we're unable to use the next biggest denomination. For example, a base-2 payment of 1111 (or 15 in base-10) sats that's one away from being rounded up to 10000 (or 16 in base-10), or a base-10 payment of 999 sats that's one away from being rounded up to 1000. For base-2, our worst-case is 2-1=1 note per denomination per payment. For base-10, our worst-case is 10-1=9 notes per denomination per payment.

This sets the floor for the number of notes per denomination to target_per_denomination * (denomination_base - 1). However, we need to account for potential change notes to calculate the ceiling. Each denomination may have up to denomination_base - 1 notes as change due to having a total amount of ECash that can't be evenly broken into the denomination buckets.

So I think max_notes_per_denomination instead is (target_per_denomination * (denomination_base - 1)) (denomination_base - 1) where the first section accounts for offline payments and the second section accounts for change. This can be simplified to the following:

(target_per_denomination 1) * (denomination_base - 1)

Where the 1 accounts for change and the - 1 gives us the maximum value before moving to a bigger denomination (the largest digit in base-10 is 9, which is 10-1).

Does that sound correct?