Rule of thumb for cooking or baking different quantities

Here's a few lists:

• http://www.backwoodshome.com/articles/kovach59.html
• http://www.ochef.com/617.htm
• http://www.gardenersnet.com/recipes/blanchingvegetables.htm

A 13" cake is large enough to be a major problem -- the center just won't set correctly before the edges overcook.

You generally want to cook large cakes at a lower temperature than you'd cook a smaller cake at; they won't rise as much, but as they're typically used for stacking, a denser cake may be preferred.

There are also two gadgets that you may wish to consider when you're dealing with cakes over 10 inches:

• Baking Strips : They cool the edge of the cake, so that it cooks slower. Wilton sells them, but you can also make your own from an old towel
• Heating Core : They conduct heat to the middle of the cake, so it'll cook faster. Wilton sells them, too, but if it's not too deep of a cake, you may be able to use a flower nail. If you have a heat safe metal cup, that could work as well; I've heard of people using a cocktail shaker. (but it's important to grease it liberally, as you want what's inside to come out cleanly so it can be used to plug the hole).

I can't help with the gas mark, as I don't know the British system.