Add all their stats together and see how many 0.2's they need to reach a total of 3x the next star's quality.

If your player has skills 97.8, 96.8, 95.6, his next star will be at 100 quality.

His total is currently 290.2, his next star will be when his total is 3x100 = 300.

300 - 290.2 = 9.8, so he needs 9.8 points to the next star.

Each skill point adds 0.2, 9.8/0.2 = 49, he needs 49 points to the next star.

From one star to the next is 75 points.