The 300 yd is 1.5", and so on, but with a 3-12X scope you'd likely have a hard time seeing the "mothball" on a score target. For load development, sighting in, and similar shooting there are targets designed for use with lower powered scopes that are sold in most gun shops or stores that carry guns and ammunition. You could also do a search of NRA rules for high power rifle to see what bullseye size is used on their targets.
It'll likely involve using various targets to see what works best for you too. I've seen guys using stick on fluorescent dots as aiming points, but I could never get comfortable with them.
