I just through together a very simple gem calculator. You start by selecting the quality of the gems you are starting with, the quality of the gem(s) you are trying to make and the quantity desired. You also have the option of inputting AH gold prices for the various materials. The output is the required mats and gold you will need.
Let me know if you find it useful and/or things that you would like to see added to help you out as a quick reference when crafting gems. Thanks!