Oh lord. You need to think about quantities first. It's all to do with volume rather than size. So from what I can remember if you want to scale up a recipe for an 8 inch tin to a 10 inch it's actually one and half times the recipe. And for a 12 inch it's two and a half. But that's round tins not square. I think the calculations are the same for a square but the measurements are 7, 9 and 11. Gawd its a long time since I've done this. I used to make wedding cakes. But the way you work it out is to fill the tins with water and calculate it by volume. You'll need a calculator for that.
Scaling up the cooking times isn't so logical. Keep the temperature the same but protect the outside of the tin so the edges don't burn. Tie newspaper the same height as the tin around the sides. If you're scaling up an 8 to a 10 then I'd probably up the time by a third but be keeping my eyes peeled from the time the smaller one should be cooked.
Does that help any?! I'm 100% sure on the quantities on the round tins, 95% on the square tins and winging it a bit on the timings.