A: You should signup for a free account to get a better idea of this. I'm not saying that to be difficult I promise but the credits work in a way in which you can use each of the aspect ratios for 1 image, 1 credit at certain dimensions, the steps you choose to use add to the credit amount as do the higher dimensions of an image.

But what I can say is that we don't bump things up massively, the increase in decimal numbers. A square 1536x1536 at 20 steps is 1.8 credits, 768x768 is 1.3 and 512x512 is 1. If you were to use maximum 80 steps which really isn't necessary for most of the samplers, you would be looking at 1536x1536 for 3.6, 768x768 for 2.6 and 512x512 for 2.

Hope that helps!