There is no place that you need to enter the plan rate itself. If you mark the Maximize box in TurboTax for a SEP plan, TurboTax assumes the maximum permissible base plan rate of 25% which works out to the maximum permissible SEP contribution for the self-employed of 20% of net earnings. (Net earnings = net profit minus the deductible portion of self-employment taxes.) Any contribution less than the maximum permissible implies a base plan rate of less than 25%. If you make a SEP contribution of less than the maximum permissible you simply enter the amount contributed and do not mark the Maximize box; it doesn't matter what the plan rate actually works out to unless you have eligible employees for whom you also must make SEP contributions.
Thanks, this is very helpful. So there's no fixed % rate that must be used year after year, right? I can decide which rate (or amount if only me) as long as it's not more than the maximum allowed, and this can vary year to year?