My horse in worst cases repairs 2 rounds out of 10, arrows on my archer usually get repaired 7/10 times. And those are not just numbers from the top of my head
I don't know either the formula.
But i do know that you can only carry a single horse every round, while you can have two quivers of bodkin arrows per round (wich effectively double the chance of your bodkins arrows breaking).
I don't know how many arrows your character used when you did your research, but if you carried two, and one (since it's the max) broke 7/10 rounds, then, maybe a single quiver would have needed repairs 3.5/10 rounds. So effectively, less than twice as much as your horse.
So doubling the upkeep on arrows and removing extra breaking chances for them, would then be an increase in archery upkeep.