There are an incredible amount of variables that influence how much power a car can put on the rollers yes, and most of the things you mention there would influence the amount of power to the wheels and therefore the calculated flywheel figure would vary also, even the way the car is strapped down can impact figures.
To all intents and purposes a calculation of 18-20% is pretty accurate, but the bottom line is that if you want an accurate flywheel figure you need an engine dyno
As I said before customers want the big number sheet, how many people post their whp dyno sheets ? Not many, nobody wants to say that their 370z with bolts on and a map makes 250hp but that fact is they do
You would have to ask Jez what the Dyno Dynamics uses as a calculation I would guess its close to 20%
These big rolling road meets should be treated as what they are , a bit of fun for like minded people who have an interest in cars to meet up and have a crack not as a means to get accurate data from your car , chances are the car wont be strapped correctly or be set up properly before the run is done , this all takes too much time if you did it all correctly you would get 4 cars done in the day and you would only provide the measured wheel or hub data