Your calculation is correct. You do need 24 to get to next star level at 25. Using my method its:
Defence 22 (need 3 for 25) = 3x5 = 15.
Attack 25. (need 0 as he's already there).
Physical 23.2 (need 1.8) = 1x5 + 4 for the extra .8, = 9.
Total required: 15 + 0 + 9 = 24.