The key thing to remember about the ratio calculators (mine included) is that they provide you with a theoretical maximum speed. They do not consider the power requirements to overcome friction and aerodynamic losses. The various power calculators do consider this, so it is useful to use both tools.
That's it.
I find it's most useful to use a power calculator first, to figure out about how fast I can go with the net power available. Then I figure our what ratio I need to get there.
Chalo