I do a trial run with pencil even ink on a 1040 form, punch the numbers with my calculator, then when satisfied use the PDF Web fill version and print out two copies. I use the previous return as a guide for the current tax return, if I have a question I read the 1040 instructions, go to IRS web site, or ask a friend, hell if I screw up I figure the IRS will figure it out for me. Let them do the work.
And one time I did screw up, got a letter to explain a deduction from a previous year's return, I put the deduction on the wrong line and replied by mail with copies of the checks to support my deduction and problem solved.