employer SEP for S-corp

A 100% S-Corp owner wants to make an employer SEP contribution, which can be no more than 25% of W-2 wages from the S-Corp. Box 1 of his W-2 includes $10,000 of health insurance premiums paid for by the S-corp, but the premiums are not subject to fica or medicare tax. Is the 25% employer SEP limitation based on Box 1 wages, or is it limited to wages subject to fica and medicare tax?