The larger your income, the more you have to pay. At a certain level, you are required to pay 110%. I do this for quarters 1, 2 and 3. When quarter 4 comes around, I look to see if my income will be the same or less than the previous year. If my income will be the same or less, I true up my Q4 payment to be 100% of the tax I owed in the previous year. If my income will be more than the previous year, I pay Q4 at 110%.
The alternative is to always pay what TurboTax tells you, and you will have a nice refund or an amount to apply to your next year's taxes.