I got the equation from a Bachelor Thesis. Unfortunately the professor does not have time to explain neither the equation nor my calculations to me. That is why I am asking here.
I am by no means capable of coming up with this equation on my own.
The 1700m are from flyAPG. But you are correct it is the TODR. So I figure from Lift Off to 35/50feet takes about 200m. So the ground roll is about 1500m, which at 18100 pounds seems realistic for me.