I don't do JM's for a max effort movement, I do them after speed bench for 2 weeks, then go to skullcrushers for 2 weeks.
I have worked up to a triple with 315 on JM's, and I have benched 475. I don't know what percentage that is, don't have a calculator handy and I ain't that smart. My weights in the JM have gone up dramatically since I started doing them, so don't sweat it if you think your numbers are too low right now, they will come up quickly.
Also, everyone does them different. I have been told it is 85% press and 15% tricep extension. I try and keep it there. But I have heard from some people that if you are doing more than half your bench, you are not doing it right. Each to his own. I can do a skullcrusher down to the bench behind my head with close to half my bench, so I don't know that the half your max theory holds much water.