Your calculation is correct.
As kbob and me wrote, the 0.7 factor is only a rough estimate but the 250 KV motor does seem too fast.
BTW you can use the calculator boisrondevens posted by entering the diameters instead on tooth count in the reduction fields:
Reduction1 - primary: 63 Teeth
Reduction1 - secondary: 700 Teeth
Reduction2 - primary: 1 Teeth
Reduction2 - secondary: 1 Teeth
Reduction3 - primary: 1 Teeth
Reduction3 - secondary: 1 Teeth
Avner