Error X sight radius / distance (all measurements in inches) will give you the increase you need.
It looks like a .320" would get you very close, with the rear sight setting as it is.
I would be inclined to shoot the rifle at a further distances, at least 50 yards, to a better idea of what you need. As it is, it looks like one of the .343" versions would get your rear sight more towards the middle of its' range of adjustment.
A low point of impact can be dealt with by filing a front sight that is too tall.
