What is the +To Hit% Cap at?

A. The To Hit% Cap is simply your miss chance against that particular mob. The melee miss chance against a mob depends on the level difference between you and the mob (5% at equal level, +1% for each level the mob is higher than you), as well as the difference between your weapon skill level and the mob's defense level (+0.04% per skill point of difference). Therefore, if you are at level 60, the miss rate (and subsequently the To Hit% Cap) would be:

Level 60 Mobs: 5%
Level 61 Mobs: 5.2%
Level 62 Mobs: 5.4%
Level 63 Mobs: 5.6%
Boss level Mobs: ~8%