HTB, that was corect. The cruical measurement was, IIRC, bridge of nose to chin. Even though I don't have the smallest conk I used the smaller Q mask.
Interesting what you say about the G-Helmet. It must have almost been a peer-fashion think or a refusal to supply the Mk2s.
In Cypus in the early '70s an increasing number of us acquired the Mk 2. Far more comfortable in the higher temps at low level. Initially we also carried the g-helmet so you could get your head into tight spaces such as for fuse changing and astro shots. I found astro was no problem nor were many of the other areas. I can't remember but I think I stopped carrying it in the end. I think on one trip I had to go down on the door and up to the JB343 (?) and 11P behind the nav crate. I think I had to dispense with a helmet altogether.