You must (and I’ll repeat) you must actually measure your skin waist or the finished waist from your best fitting slacks to get the real waist measurement you are comfortable with.
Measure the Outseam of your best fitting slack’s – This is the outer seam of the slacks measured from the top of waistband to the bottom.
Measure the inseam of your best fitting slacks. Place the tape at the crotch where the four seams meet and measure to the bottom of pant.