Oxygen system information required for BoB Spitfires and Hurricanes please.

Oxygen systems in aircraft

A friend of mine is doing some background research on O2 systems in BOB aircraft. I wonder if our knowledgable community can help me with some information:

BOB spitfires & Hurricanes have a butterfly value operated oxygen regulator. The regualtor could be set to deliver a variable rate of oxygen flow based notionally on altitide. ie. the pilot would select (say) an altitude of 15,000ft on the regularot & oxygen would flow at some rate; the pilot could change the setting to (say) 20,000ft & oxygen would flow at a higher rate, given the lower pressure at height. Does anyone have any techincal information on what the actual flow-rate was in the regulator at various settings?

Thanks for any help, cheers, MP