Motor KV Calculation vs. Reality

[notger]

Hi,

my Question is about how to calculare the right Motor KV, based on the fact of the bike and the reduction.
I know that it can be made really scientifiy, cause of motor characteristics an all that stuff, but i actually would just find out some "Rule of thumb" here.

So lets take an Example:
My Bike is an 26 Inch wheeled Downhiller:
the whole thing will get an mid drive with an belt reduction

Battery 15s: 45-60Volts from full to empty
Motor Alienpower systems(turnigy) 8085: with 80KV
Motor Reduction from Motor to CrankChain Ring: 12:1
From Chainring to sprocket 40t->18t: 2,2:1
Wheel circumference: 2100mm

So would i "Just" calculate that now the bike would be 113-84 Km/h fast (batterie full to empty 4V-3V/cell)

But sure, there is energy-waste on the way from the Motor to the Wheel.

So i just take as my Thumb of Rule 20% Loss now, then the bike would be 90-68 Km/h fast, wich would sound much more drive- and reasoneable.

What are Your ecperiences in Calculating teh Right Motor KV for your Projects ?
I guess there is still alot of Trial-and-error needed to find the right Motor.
But id like to find some kind of guidline to order the Right Motor KV for My project.

Thanks in advance

Notger