VIC, we first set up a 315 foot long straight [1/16th mile]. Our hatch back cars are lined up with the tips of the raised tail gates even with the imaginary plane that the model has to cross. I am with helper #1 who is in charge of telling helper #2 when to start the stop watch. Then all helper #2 has to do is stop the watch when the plane flies past the finish line. If something was to go wrong with the plane, everyone has a car for protection. If both helpers have stop watches, then you don't need to do a complete lap to get another clocking, just do a U turn and let the helpers reverse roles.

CH, for what you're using the math for, the formulas work well. My point is that when you get into the 170 plus range the slightest wrong assumption in any formula is going to be amplified. I have a Diamond Dust that on paper should do 195 but it actually does 177. There's no substitute for knowing for sure and it is amazing how repeatable the clockings have been with my 2 helpers. No matter how hard I've tried, I can't get them to make my planes seem any faster.