I have been using a similar setup with my Logan stirrup pump for a few years now. Once a year I strip, clean, check/replace the O rings and tophat, relube and then pump it up to give me +- 11.4fpe with webley mozzies. I agree 11.8 is a little high if your using a heavy pellet as with a lighter pellet you'll be over the top. Better be SAFE than sorry
Cheers and safe shooting
Gary