If you are a sole proprietor, the maximum amount you can contribute to a SEP is approximately 18.59% of your income from self-employment (20% minus a deduction based on one-half self-employment tax). If you hit the maximize button in Turbotax, it will calculate your maximum allowed amount.
If you have no other contributions based on wage employment (like a employer 401 (k) plan), you can increase your contribution starting in tax year 2019 by starting a solo 401 (k) plan, which has a larger contribution limit. It is too late to do that for 2018, however.
