For anyone that comes across this in the future:
I called Ruger, very quick and helpful but wouldn't say anything concrete. Hours searching forums etc I found two guys that built custom No1 rifles. One said he tried for 8-10 thou clearance on the breech block, the other tried for 5 but "wouldn't cry over 10". Both were very clear to say they weren't talking about headspace but breechblock clearance; this matched what Ruger said, that you cant measure the headspace with feeler guages down the breechblock. I started searching block clearance but found little so I moved to headspace search. Neither worked, I found what I needed after scrolling through approx. 50 pages of the forum.
THE ACCURATERELOADING.COM GUNSMITHING FORUM had the info, I'm not searching further