The goal with drop tuning is always consistency - you want the drop-tuned guitar to FEEL the same as your standard-tuned guitar. While everyone will have a different preference, I find a good starting point is one gauge per whole step.
So if I like my 9-42s for E standard, then I would start with 10-46 for D standard.
If I like 9-46 for Drop D, then I'd start with 10-48 for Drop C.
Make sense? That's a good starting point, and you can adjust again with your next set of strings if it's not quite right. Ernies come in split gauges now - I use 9.5-44s for my Eb standard guitars.