I'm going from memory here so...
I recently put a Maccari kit in my 50s. I do remember he had a sub 12fpe kit but I used his regular one. That one gave me a few less fps from factory, but it made the gun smoother shooting and less twang/buzz. The 50 is not that powerful a gun to begin with. I believe my factory spring had 3-4 coils of preload and the new Maccari spring had 2-3, again, from memory.
Anyway, I would recommend the regular Maccari kit and seal. My backyard range is 22 yds, but I'm dead on from 15 to 30 yds with the ftt 11.42 in .20 caliber.