The maximum permissible percentage of net earnings by a self-employed individual for a SEP contribution is 20%. Because a SEP contribution is an employer contribution, the SEP contribution itself reduces the amount of net earnings which would otherwise be subject to the 25% limit. The calculation is described in IRS Pub 560 Chapter 5: 0.25 / (1 + 0.25) = 0.20
https://www.irs.gov/pub/irs-pdf/p560.pdf
