How much Memory does a SQL Instance need ?

This is a question that haunts us from time to time in our farmed SQL environments, it is often overlooked at the design phase, and a 'finger in the air' approach is used to arrive at an affordable figure. My standard answer to the question is  "How fast do you want your Instance to be ?". There are vast amounts of information on the web on this topic but none offer a simple way to get an answer.

I stumbled across this little gem of information on technet last week, it is a way to find a starting point for the Max Memory Setting for your Instance :

 ([Stolen pages] + [Reserved pages])/ 100

Line 20 of the following article : http://technet.microsoft.com/en-us/library/ms178067(v=sql.105).aspx