An increase from saami standards (look here
https://saami.org/wp-content/upload...FP-and-R-Approved-2015-12-14-Posting-Copy.pdf) an increase from 18,300cup to 21,500cup would be 17.5%.
I suggest that if your gun only has a 18% safety margin it isn't acceptable in the first place. SAAMI proof loads are 25% over standard and designed for the weakest guns made in those groups so a proof load of 25% over standard should not blow up a 1949 J frame 38 special nor a 1926 K38.
But then I am not a huge +P fan for most guns. I think that for most people and guns, especially smaller, lighter, concealed carry guns the increase in velocity doesn't pay off in the cost of recoil recovery and accuracy. I also have my doubts about the wisdom of practice with standard ammo and then carry +P ammo. When the chips are all on the table, I want everything to be close to what I worked with. Don't need no more factors along with the stress.