Business & farm

TurboTax will calculate the maximum amount you can deduct if you check the box for the Maximizer.  If your entry is too big, you will be prompted of the excess contribution.  See Below,

If you overcontribute to a retirement plan in any tax year, you have put more money into the plan than you are allowed to based on the circumstances of your return.  You must withdraw any excess contribution by the due date of your return, or else you may have to pay a penalty. Adjust the contributions on your tax return by any amount withdrawn.