I wouldn't go lighter than 10 1/2 pounds including scope if you plan on running a 500 grain projectile at 2300 fps or above. If you were using the .450 G & A Short running a 500 grain projectile at 2200 fps then I'd aim for no less than 10lbs including scope.
My current 450 Rigby Rimless is 10 lbs 10 oz, iron sights no scope. Controllable with 500 gn bullets at 2350-2425 fps for follow ups. I wouldn’t make it any lighter.
The 460 G&A I’m working on now should come in about 10 1/2 lbs - no less - planning a 500 gn bullet at about 2300 fps. The main variable to achieve that weight will be the barrel contour. Plan to call Douglas to get an accurate weight of one of their 23” barrels chambered in 458 Lott and adjust the countour based on that. A few ounces will be added by peep sight, barrel band front swivel, NECG Masterpiece Banded ramp FS. Haven’t decided yet whether to keep factory bottom metal with a different follower, spring, magazine box. That decision may add a few ounces as well.
I want to make sure that the rifle retains balance when mounted and aimed. That’s critical IME for first and follow up shots.
