25% compensation, the compensation has to exclude:
1. 1/2 of self employment tax 7.065%
2. the contribution itself.
So for 100000 net profit, excluding $7065 SS tax (employer portion), the left over is $92935.
sep contribution = (92935 - sep contribution )/4 see rule 2 above.
therefore sep contribution = 92935/5
More generally
Sep contribution = (net profit * (1-7.065%))/5