I understand that the speed I get after i do all the calculations will be top speed on a flat horizontal surface. Am I correct?
Nope, read again

hehe.. sorry couldn't resist.
No,
you set what conditions you need (weight, groundstructure, inclination...)
and then use the formulas to make your guess..
If you read closely, you'd noticed that the last function is a simplification ONLY.
I see those ebay controllers mentioned a lot. Has anyone here actually used on in their kart? Seems like Karts would be more abusive than a scooter...
And whats with all the winking?
As far as speed, at a minimum, you can always go 4mph because thats how fast your legs can push it.
The abuse of the kart is done to the motor, not the controller.
so it will die loong before the controller will.
it will get hotter (or precise warmer) than on a scooter, because it'll run a denser powercycle, but still good enough to move a kart for a long long time, much longer than your average battery-pack is able to provide the power.
the winking is a a habit,
someone once told me he could tell the mood I'm in by the number of '

' in my posts.
live with it or die trying
'sid