SMLE Mk III* Reload Information

[Sunray]

Start by slugging the barrel. I'm not sure if Australia had somewhat vague ideas of what bore/groove diameters meant or not, but there are lots of .311" and .312" bullet around. And that assumes your rifle is Australian made. Mine is not.
Prvi uses .311". So does Sierra. Hornady uses .312".
Then just work up the load using ADI's(who make several powders for Hodgdon) data and you'll be fine.
"...Be wary of using equivalents..." Forget equivalents altogether.