Used the math, went out and rode next to a stock bike with an accurate spedometer and was still off. Keep upping it until it was matching.
Stock Japanese bikes do not
have accurate speedometers. They're all 5-7 percent fast, meaning when the speedo says 100, you're going slower, like 93-95 or so.
However, modern cars have very accurate speedometers, regardless of make. My not-so-modern 2003 Vette exactly matches every road radar I've ever been thru, and with the heads-up display I never have to glance down at the speedo to confirm. Always identical. So....use a car, not a bike.
Work out a signal with the car driver, like when they're holding a nice steady 70, hold their fist up. Keep is simple so they don't wander around 70 trying to tell you what the speed is via arm motions and exaggerating mouthing of unhearable words. Tell them you only care about 70, and show you a fist. Nuttin' else. And DON'T enlist your galpal or wife. A squabble will surely ensue. YOU
: "What is it about a steady fukkn 70 you don't understand?!!!" HER
: "Shove that speedohealer up your asssssss!!"
Why 70? It's close enough to the lower speeds when an LEO might be running radar (neighborhoods) and halfway to 140, which is fast enough not to be looking at a speedo anyway! Point is, 70 negates error in either direction, and is a typical highway speed to worry about radar problems.