Hi there Sugarmama
I do a weight to volume measure so measure the volume of the cream. In metric that's easy but I don't know the conversion for imperial measures...However...I have developed a little spreadsheet for just this purpose. It is called "The Ganacherator" and you can access it here
(newer version of Excel)
(older versions of excel)
It allows you to put in the size of your cake and the number of layers and tells you how much ganache you need, then tells you the weight of choc and vol of cream (for both white and dark/milk chocolate)
Fortuitously, I just made some adjustments so that you get your weights and volumes in oz. and fl.oz.
I'm from Australia and we use grams and mls. which is not so useful to you!
My email is on the spreadsheet if you have any feedback or questions.
Hope it helps!