If someone files a 1040 with a schedule C, when discerning the limit of annual additions for the owner, must this person reduce earned income by half of the self-employment tax? What about the 20% limit?

See IRS publication 560.

Reduce SEI by 50% of the SE tax. The maximum contribution is effectively 20% of the resulting number, but I would prefer not to call it a "20% limit." The limit is 25% of compensation.