You can also use Length * Width * Height * 0.4 to give an approximate amount of the oils you will need to fill the mold.
For example, the Bramble Berry 10" silicone mold would be 3.625 * 2.25 * 10 * 0.4 = 32.625, which is just slightly over 2 lbs of oils. But I also have an extra mold handy, just in case.
If you see a recipe that is given in percentages, or can convert it into percentages, you can scale it up or down yourself. If a recipe is say 50% Olive, 30% Palm, 20% Coconut (<---totally just made that up, use whatever recipe you have), then you would multiply the total weight of oils by each percentage to get a scaled recipe.
So to fit my BB mold from above, it would look like this:
50% Olive * 32.625 oz = 16.3125
30% Palm * 32.625 oz = 9.7875
20% Coconut * 32.625 oz = 6.525 oz
Bramble Berry has a calculator that will do that for you too, in case you're not good at math:
http://www.brambleberry.com/Pages/Lye-Calculator.aspx