scope
If you want to test your crosshairs, and your tracking for perfect vertical adjustment it is quite simple. Take a big sheet of paper, at least 3 feet long. Put your target up at 100 yards.. Hang a plum line from the top of your target. Draw a straight line with felt pen and ruler, following the plum line. Now make a crosshair near the bottom for an aimpoint. Zero your rifle so your hitting this crosshair on the line, your rifle being a custom build should be very precise. Once you have it zeroed, crank your scope up a lot, say 15 inches or so, more is better. Now shoot your rifle at the same aimpoint. If your shooting to the right or left of your plum line(15+inches high), then your crosshairs are crooked, and your scope will track left or right as you click it up. More clicks will be more error.