As an easy to understand rule, what effects the elevation is the horizontal length the projectile travels, as this is the part theat gravity, working perpendicular to the earths surface has to work on.
With this in mind, think of a right angle triangle with a long side of 270m, and an angle of slope of 30 degrees. The target will be 135m higher(or lower) than you, and the horizontal distance will be about 234m. If your rifle was zeroed for 234m it would be pretty much exactly on target shooting at that 270m 30 degree deer. The other thing to remember is that the wind has 270m of effect, so do your wind calcs on the actual distance the bullet travels, not the horizontal distance.
As others have said, it doesn't matter if you are shooting uphill or down - the effective distance for drop will be the horizontal distance, not the slope distance.
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