How do you measure birefringence of thin rock sections?

Im getting a bit confused about how to measure the birefringence of minerals when using a microscope in my geology cl. Obviously the yser is in so that its not in plain polarised light. Then were given a birefringence chart and asked to determine the birefringence of a certain mineral in the slide. When the stage is turned, the colours change through extinction (black) to whatever colour. So, when you need to determine the birefringence colour (and find out what order its in), is this the maximum birefringence ? If so how do you measure this ? Im thinking that to measure this, you turn the stage a certain amount from the extinction, and then that colour is the maximum birefringence and the one that's use-full. The problem im having is that i don't know what im looking for, it all just looks like a load of changing colours, can anyone help.