The method I used to derive the rough 4:1 ratio (which is not precise) is by comparing a top speed to rpm, and the tire size. If you know a stock on stock banshee does 78mph at 10K with 20 tires, you can determine the final axle ratio including all gear reductions at the crank, trans, and chain sprockets. Obviously I have a more accurate way of doing this but this is just to better understand the math.
What I see is say 200ft/lb of force at the back axle/tire size in ft to determine moment force on roller. This works out to 200/(20/12) or 119.8lbs of force.
What I am getting at is all these damn inertial dynos floating around testing power but without proper ratios in place, it is a GUESS! I saw this on another dyno console that said "standard calculation", asked the operator what it was, he said, "dunno but it works". Bottom like is how the hell can they be anywhere close to accurate?