Speed Drop is a way of estimating drop in a MIL reticle so that to hit a target at 300 yards, you hold 3 mil of elevation. To hit at 550 yards, hold 5.5 mil, etc...
This calculator will ONLY work if you are using a MIL reticle.
Fill in your calculated drop in mils. If you haven't already, you can calculate that here or here.