When I make gnocchi, I plan on a 1:1 ratio of russet potato to flour however I add a little bit of flour at a time until I get the consistency I want. I would advise against putting the entire cup in at once because, depending on the moisture in the potatoes, you can end up with a very dry, heavy dough.
If you're unsure about the dough, take a small bit, form a gnocchi and cook it to ensure it won't fall apart in the water. If it holds up then you're good to go.
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