Forgive me if I've misread you. But, if the battery is supposed to last 3 months in standby, I would expect the battery to drain by about 1% per day. Yours losing 20% in 21 days seems pretty consistent with that. This is the expected behavior, right?
As for the 30 hours in dive mode, I don't really know how to compare that against your 50% battery loss in 7 hours of diving, but it sounds like it's draining a little faster than expected. Do you have any sense of how much time you have the computer in dive mode, while not actually diving? If it's about an hour per dive, then that plus the 7 hours actually diving would add up to 15 hours, or 50% of the expected time. But, that sounds like a high estimate, to me anyways