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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 

https://www.irs.gov/retirement-plans/self-employed-individuals-calculating-your-own-retirement-plan-contribution-and-deduction