First, I don't think you can replicate the FGMM, because it's most likely a non-canister powder.
Second, if you want to improve it, start with an OAL that's appropriate for your percision build, short throated rifle, not the mass market.
As a handloader, if you can't come up with something that shoots better in your rifle than a factory load, you are not doing it right.
I suggest you start in the sticky thread named "Good .308 Loads", where members shared a couple hundred proven loads. See what powders people like with your chosen bullet, put 4 or 5 of those powders behind your bullet, Start low, and just OFF the rifling. and see what your rifle likes. Take the one's that look promising, then work up, and if necessary then work in.
I suspect you will have more success going then route then trying to replicate some factory load.
