Setting up M4 mag for non 308 cartridge

[bigfish27]

I am trying to get a M4 magazine setup to feed 224 Weatherby rounds in a 308 magazine. COL is to long for a 223 magazine.
One cartridge feeds fine. Two or more the cartridge feeds but the effort is really high. I think the answer is trimming the spring. Is this done by trimming small until it works or is there some way to get in the ball park quickly?
Thanks
Marlin