303 Brit SAAMI cartridges are registered from .054" to .064".
303 Brit SAAMI chambers are registered from .064" to .071".
95% of the brass in the real world is .062" or less.
If I were to chamber a Ruger #1 in 303B, I would set the headpsace at .062".
I am really sick of the range of 303 Brit Enfield No. 4 Mark I chambers, bolt heads, and thin compressible bolt bodies with lug in the rear.
The Ruger #1 is the best action I come into contact with, and tight headspace would be nice.
The 303 Brit brass pushed to the threshold of long brass life and backed off a safety margin in a #1 action, should exceed the power of the weaker 308 cartridge pushed to the edge and backed off a safety margin.