modify has their numbers posted in a link airsoftparts has posted with the springs. the numbers are still guesstimates unless you use the exact parts in a similar gun.
http://www.airsoftparts.ca/news/modifyspringtest.pdf
I use the s100 in my m4 but it's shooting a little soft at 330fps dude to an air leak I'm hope to resolve with the madbull LED hopup and new rubber.