I'm sure client also states that they've carefully reviewed the return and that there are no errors on it. In fact, the only line item different between Lacerte and TurboTax is Line 17 on Form 8960. It must be that one software or the other is simply incapable of multiplying by 3.8%. Alas, it is beyond us to determine which one has multiplied correctly.

Assuming the Line 16 to Line 17 multiplication and the other worksheet basic math is actually correct, neither Lacerte nor TurboTax are calculating anything. They're taking whatever input they're given and plunking it on the lines the person doing the input told them to. Look at the 8960s and see which numbers are different. Then figure out why. 

FWIW, I verify on all of my returns via an Excel file that takes the numbers I expect to flow to the 8960 and runs them through the math of the form. My Excel file says that for every return I've done this year, Lacerte did the math right if I got the input right.