I don't think it's ideal neither, but it's the least confusing UX I could think of.

Users will probably only want to give a certain npub either more or nothing at all (in the latter case it should then stay fix on zero percent btw)

Using your Top-to-Bottom idea seems confusing to me and involves calculation and forethought, things I really want to avoid. Or I might have misunderstood it. Is there a particular order to the splits?