The load for your gun is in Graeme Wright's book.

My rifle is written up on pages 86 in his book, as (Quote)
"Using my records the very best groups that I have obtained at 50 yards are as follows.
A H&H 500/450 x 3 1/4" - 0.8 inches (4 shots) "

Believe me, the gun shot as well as the book says all of the time so I sometimes
wonder why I sold it.

It sounds like you are on a winner - try changing the SEATING DEPTH of the bullet
- I have found this can make a difference, especially when you want to make small differences
once you have the correct velocity / rough regulation like you have.

Anyway, good luck.

500 Nitro