Hi Tim - I am also using odd cake pan sizes. I am sure the experienced bakers can help you. There are tables and other stats in the cake bible which provide you with factors with which you can multiply the ingredients.
There a couple of posts on this but you really need the assistance of some of the maths/baking experts here such as Julie
I am still battling to grasp this concept so it can apply to all cakes. I am still studying the Cake Bible - Rose has included several factors for various cake sizes in a table but I am not sure if they apply to all recipes. I don’t feel confident enough to advise you. Do you have the Cake Bible?
I am not using US standard 9 inch pans which are thus far unavailable locally. I am trying to buy some but thus far no luck in locating. It makes baking so much more difficult.
I do wish the pro bakers would compose a sticky which we could all have as a guide for non mathematicians! The site need this particularly as Rose has a growing client base outside of the US.