For easier split amount entry, show the amount that is left to be applied. This is quicker and cleaner than calculating that amount based on gift amount and what has already been applied (also because the split gift screen visually is a bit of a mess).
I would be fine with the amount left to be applied, or the difference between the total applied and total expected, in addition to what's currently on the split gift screen. It's not mission-critical, but having the calculation in Database view was such a useful tool, that I don't understand why they'd skip it.