When testing a deck, what % of rejected hands is acceptable?
It really depends on the format, your deck, and the current meta.
It also depends on why you're mulliganing:


If you need to mulligan due to land percentage (too many/few), you need to fix your deck.
If you need to mulligan because you have no creatures, it might be an issue, but that depends on how much you need them [Which in turn depends on what kind of deck your opponent is running]
If you need to mulligan because you don't have a certain card in hand... How important is that card?  [It's probably OK to mulligan for a one-drop Enchantment* or a Layline (Which aren't pauper-legal), as long as it's worth losing the Card Advantage]


The best way to test it is to just build the deck, and see what happens.

*I have one Modern deck that sometimes has to mulligan down to three or two cards in hand.
As long as I'm not up against an aggro deck, I can still win with an opening hand of [Island, Quest for Ula's Temple].