Your deductible self-employed SEP-IRA contribution is the lesser of 25% of your net earnings from self-employment or $55,000 (in 2018), as adjusted by the special handling described in Publication 560 (https://www.irs.gov/publications/p560#en_US_2018_publink10008841).
As you will see from reading the sources below, the upper limit for self-employed SEP-IRA contribution is going to be about the 18.587% that the poster in the second link below describes (or $55,000). This is the calculation that TurboTax uses when you ask TurboTax to maximize the SEP-IRA contribution.
Go to the link above and search for "Deduction Limit for Self-Employed Individuals". Read this paragraph and note that your contribution to the SEP-IRA affects your net earnings, which creates a messy situation in that the calculation of the 25% of net earnings is going to vary. You can go to the tables referred to in the IRS text, or try reading these two discussions (note, they may not have been written for 2018):
