In my opinion the time is of too big value in tactics trainer. You should score more then 20% if you think more than twice the average time. Also you should score 0% if you play the first moves right, but then make a blunder that would make you lose the game or allow draw if you were winning (Here I refer won position the same as a clear advantage and draw the same as an equal position). A general advice is to focus on solving the most problems and not try to gain rating points (which isn't the same thing since you can guess if your candidate move is correct instead of calculating.)
Yep, 3/4 should be incorrect not 75%, especially when you can throw away a queen and fail to follow it up to mate, leaving yourself with a losing material disadvantage.
As pedagogic tool, the TT here is hopelessly broken as it rewards behavior that would be punished in OTB play and penalizes behavior that is rewarded in OTB play.