What I do: I'll use a short range "so much high" preliminary zero to get me on paper at the longer range,but if I want a 200 yd zero,I actually zero at 200 yds.
I'm guessing what you may be running into is another variable caused by sight height.General rules of thumb are an OK guideline.
There are ballistic calculators where you can enter specific parameters and get results that will probably be quite close. And,you can actually enter the up/down slope of your shot. FWIW, the slope is not your problem.
The short explanation,the slope is part of a right triangle. While your perceived range is the hypotenuse of the triangle,what gravity has to work with is the long leg of the triangle. That's a little shorter than the hypotenuse,and that difference in "range" is what has an effect on elevation.
If you had 10 feet of elevation change over 200 yds,the effect on target would be negligible.You can figure it more than one way,but A squared +B squared =C squared works . Hypotenuse=600 feet squares to 360000. Slope,10 feet,squares to 100. Subtract the 100 from the 360000 So the range gravity is working on your bullet is the square root of 359900...which is 599.917 feet.I doubt you measure the range that close.
Come up with your bullet "Ballistic Coefficient",velocity,and how high your crosshairs are above the center of the bore,then go here for a free calculator you can use,compliments of Hornady
https://www.hornady.com/team-hornady...alculators/#!/
There is a selection for basic or advanced..or something like that,you can enter slope,wind,etc in advanced. Play!! Change things and see the results.
You might want to look at a 300 yd zero.. Fun tool you can learn a lot from.
It will even offer you a small "cheat sheet" range card you can print off.