Working out the compression of your engine??

[5.0failane]

I've been asked by my brother who doesn't have the internet at the moment, if I could ask around to see how people work out the compressions of their engines. He has an XF turbo that has just blown out the head gasket and wants to know what comp its running so he can get the pistons machined down to reduce the compression. Any hints tips or advice would be much appreciated and I will forward on to him.

Thanks