These calculators work pretty well if the taxpayer only has W-2 wages and a little investment income. Itemized deductions start to get a little tricky because miscellaneous itemized deductions go away, and any home mortgage interest that results from a prior refinance where cash was pulled out or a home equity line of credit will no longer be deductible come January 1. In other words, those loans are not grandfathered into the new law. You taxpayers are going to have to dig into your files and allocate your home mortgage interest on a percentage basis between acquisition debt (deductible) and refinance/home equity debt (not deductible.)
The calculators break down if a taxpayer has multiple sources of income--capital gains, self-employment, rents, investment partnerships and S Corporations, trusts, and the like.