Start with factory new Remington Nitro 27's in 7 1/2 and these will serve you well for most game situations. For further back, say beyond 35 yards, I like a 3 1/4 - 1 1/4 load (1220 fps) of larger shot like 6's or 7's. For way. way back, I like a 1 1/4 1330 fps load of 5's.
Check out your powder manufacturer's web site for particulars.
Games shooters have developed a wide variety of their favorite long range load. Read posts on ts.com like "Favorite from the wood pile load" and you will identify a number of different loadings.
Don't overlook the Remington Nitro 27 new shell in 7 1/2's. It has a longer reach than you may imagine.