Yeah, leave a few minutes for study, especially for freshmen. You can look for a formula you like, or you can eyeball it. If the gpas for the term are all in the upper 3s, you can ease back a bit.
For freshmen, I always use at least 5 minutes.
3.5+ HS GPA - 5 minutes SH
3.3-3.49 - 6
3.1-3.29 - 7
2.9-3.09 - 8
2.7-2.89 - 9
2.5-2.69 - 10
2.3-2.49 - 12
under 2.3 - 15
That usually gets freshman to 3.0 or up. Then I eyeball for older guys. Generally 4-6 for sophomores, 2-4 for juniors, 0-2 for seniors.
Some coaches are less conservative, and do something like start freshmen at 5, sophomores at 2 and none for juniors and seniors. Then they typically adjust at mid-term, when needed (and it will be needed for a lot of kids this way).