Working on a falling block action I head space and install the barrel. I know top dead center of the barrel when it is attached to the receiver but also need to know it when it is removed from the action for the extractor cut. I have done this in several 'Micky Mouse' ways over the years, none of which have made me happy. Suggestions?

my 2c. This is the way I do it. I scribe the action in the exact centre at the bottom of the barrel hole. ( that is the exact centre of the hole and not the receiver) 

I mount the barrel and sort out the headspace. I made a punch from a 1/4" square steel rod, that was ground to a 90 degree pyramidal knife edge point. Using this punch, I line it up with the scribed line on the action and punch in a permanent witness mark on both barrel and receiver. That gives me a datum reference.

I do it the same way as maqabbi