How to know correct Xmx and Xms for heap allocation?

I have read to use verbose:gc to know the correct values of xmx and xms but I don't know how to read the logs of the verbose gc? Can someone help me in knowing the correct values for Xmx? or direct me to a tutorial link using verbose gc preferably a video tutorial.. Thanks.

I always take a pragmatic approach to memory allocation - fire it up with default memory and see what happens under load.

Then I have to balance versus the other requirements of the system - memory available, other processes that have to run, expected response times, etc. so I dont think there is a single magic answer.

Bill