While 4seventy and those similar opinions are all valid I think the discussion is straying slightly from my original point... That being that here in the states where a government proof house is not available and private "proofing" is cost prohibitive various means of "proofing" homebuilt guns are employed, or in many cases I'm sure, skipped over. That said, kuduae's excelent explaination I think nails the delima! Yes, the Woolwich method (which I'd never known of before) is for testing ammo it seems. That it has scientificly established that the pressure forces of any ammo can be expected to increase by the target 30% is the key to the issue.
Yes 4seventy is correct that it is 30% of an unknown but then in this particular situation it is "unknown" anyway so any attempts to increase it by any factor are also "unknown!" Increasing bullet weight or powder charge in the absence of expensive equipment and the expertise to use and interpret the results is merely conjecture and could be spot on or very far off.
This more or less proven method of increasing even an "unknown" pressure by roughly 30% should be useful in my opinion. Where this really shines is as someone else mentioned I think, that a double rifle is regulated to a particular load. If THAT load can be increased by roughly 30% by this method we at least have a reference even in the absence of knowing it's actual pressure either oiled or dry! That is the objective isn't it?