The net effect of the 15 foot rise at the target area is minimal. The effect of shooting uphill (or downhill) is that the horizontal distance is less than the line of sight distance. It is the horizontal distance that affects the strike of the bullet due to gravity acting on it. You can calculate the horizontal distance as the third leg of a right triangle, assuming your 200-yard distance to your target was measured by a laser range finder. A= square root of (C squared minus B squared) where A is the horozontal distance, B is the 15 foot target height, and C is the 200 yard line of sight to the target. You could then enter the distance A in a ballistics program to see what the zero distance would be off. Conversely, you could set you target at line of sight distance of 200 plus some, so that the distance A would be exactly 200 yards. In your case, shooting at the line of sight of 200 yards equates to a horozontal distance of 199.987 yards. Your zero would be off by the performance difference of shooting two identical rounds at the two distances. To have a perfect 200 yard zero, set the line of sight distance to be 200.012 yards.
By the way, the differences are not worth worrying about. If you are shooting at a horozontal distance of 199.987 yards, and your zero is at 200 yards, your bullet strike will be a fraction of an inch high (whether shooting uphill or downhill).
