May is plenty of time. I scored Acad Avg: 14 overall on my practice test the first time. Its been a month and a half and I'm hitting 17+s. i sincerely think the amt of time and understanding u put in for the dat is correlated with how well u do. goodluck! get to dominating that test.
my study routine:
READ and UNDERSTOOD the Kaplan book (by understand, i dont mean literally know every minuscule detail) just had the overall concepts CLEAR in my mind. did occasional problems to reinforce. then started hitting subject tests hard (still doing this). any concept that isn't clear, make note, and review. DO QUESTIONS! and lots. and know why the answer choice is right and the wrong ones are wrong. i find most of the minuscule details are retained better through doing questions. hope that helps.