25% is the base rate, but because the employer contribution effectively reduces net earnings, the base rate must be adjusted. The adjusted rate for a base rate of 25% is 20%.
Presumably you have more than sufficient compensation to maximize the employee Roth contribution, otherwise you wouldn't be worrying about the amount of the employer contribution, so you can enter the maximum permissible regular Roth contribution of $22,500 and, since you are over age 50, separately enter the $7,500 catch-up Roth contribution, then mark the Maximize box to allow TurboTax to calculate the permissible employer contribution.
