Back in the Before Times, when everything was uphill both ways in the snow, we used a 10-key tape. At the bottom of each piece of paper in the file was a number that represented the net impact on taxable income the preparer expected from that piece of paper. When you're done with your input, you add the numbers from each page-bottom on the 10-key, compare that to the Lacerte calculated taxable income, adjust for phase-outs, then affix the tape to the printed Lacerte Tax Summary to demonstrate that Lacerte came up with the number you expected.

Then for one or two transitional years, we had a fillable PDF form that worked similarly.

And now we have a standard Excel template that calculates most of the the phase-outs and limitations and mechanical stuff like NIIT and QBI and the "some of that Medicare tax goes on the Federal withholding line" automatically. Every single return uses it, even kids with one W-2.

For a couple of people with the ugly K-1s that go to a dozen places on the return, they have their own modified version of the Excel, where I can map the K-1 numbers onto 90 different ugly K-1 items, and Excel adds up the pieces that all end up on the same line on the tax return and takes them to the main template automatically. It also calculates outside basis, because a GAAP basis capital account is a tool of the devil.

Some people really like to review input, and they figure if they have the input right that the output is probably right. I like to review output, and figure that if the output is right, the input is probably right.