Hi. I'm online and happy to answer your question today.
Would ordering a 12 be fine?
No. The diameter of a GP lens will change the fit. In order to change the diameter of the contact you would need an new evaluation and refit at the slit lamp.
Sorry about that :(
K. I suppose the same goes with a base? If I'm an 8.5 could I go with an 8.4 if I had to?
that is not as much of a change if all the other parameters are the same.....you might be able to get away with that, but then again the lens may not fit well.