Take your average 8 in cake tin and fill it with water to the level you normally fill it with cake batter. Measure the amount of water you have and multipy by two (if your recipe normally does two eight inch layers). You now have the number of cups in your recipe. Now, again using water) measure the number of cups you need to fill your new cake tin to the proper level with cake batter. Divide your new tin # of cups by your old tin number of cups and you will have an approximate multiple of your recipe for your new tin.
For example: if your recipe normally makes 8 cups of batter and your new tin wants 12 cups to fill it to the right level, 12 divided by 8 will tell you that you need approximately 1.5 recipes to fill your new tin.