Mike, Most anglers don't have the equipment to record the speed at the lure. Also there can be quite a difference in the accuracy from one speed sensor to the next. All things considered the speed discussed is usually the surface speed.
Different lures run with better action at different speeds.
The best way to start is to watch the action of the lure next to the boat before running it down the rigger. This unit regularly posts a slightly slower speed than my surface guages. I hope this helps. If all else fails - change your speed. Good Fishing, Capt.