The amount you want to use with this and many of the other calculators out there is not your gross but your Adjusted Gross Income. This would be the Box 1 wages from your W-2 as that figure is your gross less your pre-tax deductions 401k, FSA/HSA and Section 125 pre-tax health deductions.
Yup - assuming your income is fairly static each year, last year’s AGI would work fairly well.