Does SEP RA maximum contribution change due to what?

You have to reduce the Net Profit by the deduction allowed for one-half of your self-employment taxes.  

 

So check 1040 Schedule 1 line 15.  You have to deduct that amount from your Schedule C Net Profit.  That will give you the allowed contribution for self employment & 1099NEC income.

The calculation for a self-employed person for a SEP IRA contribution is different than the straight 25% of net income.

 

Per the IRS:

Plan compensation for a self-employed individual

To calculate your plan compensation, you reduce your net earnings from self-employment by:

• the deductible portion of your SE tax from your Form 1040 return, Schedule 1, on the line for deductible part of self-employment tax, and
• the amount of your own (not your employees’) retirement plan contribution from your Form 1040 return, Schedule 1, on the line for self-employed SEP, SIMPLE, and qualified plans.

You use your plan compensation to calculate the amount of your own contribution/deduction. Note that your plan compensation and the amount of your own plan contribution/deduction depend on each other - to compute one, you need the other (this is a circular calculation). One way to do this is to use a reduced plan contribution rate. You can use the Table and Worksheets for the Self-Employed (Publication 560) to find the reduced plan contribution rate to calculate the plan contribution and deduction for yourself.

For a self-employed individual, the maximum contribution is 20% of net earnings, not 25%, because the SEP contribution itself effectively reduces the compensation on which the base rate of 25% is being applied.