If you do not have a professional levelling device at your disposal, you can easily help yourself with a simple hose level:
This technique is absolutely precise, very cheap and was already used by Jesus and his carpenters. All you need is a transparent hose filled with normal water (ø 1-1.5cm; length: so that it hangs nicely).
You use your first screw hole as a reference height. Now hold your tube filled with water (and closed) to the middle of the drill hole / screw axis. Hold the other end of the hose at the position where you want to mount your second screw. Now open both ends of the hose and wait until the water level has settled. This level corresponds exactly to your reference height – law of physics!