At your ranges CI, it has a negligible effect. Basically, it's the Barometric pressure that effects the bullets drop. With the air being thinner, the bullet is experiencing less resistance. Gravity is still a constant and it's downward rate of speed is the same, but with less air resistance its forward path is easier. The higher the altitude, the higher your Ballistic Coefficient is going to be. But out to 300 yards with your setup the difference would be less than 1/8 of an inch (educated guess of your muzzle velocity and BC calculations). Now when you start poking out to 600+ is when you will have to dial up less than you would at say 2000 feet ALT. At least noticeably more.