The Hach 5B is accurate enough for 99% of applications. It reads in GPG. More accurate test kits read in ppm. 1 GPG = 17.1 PPM.
So... if your Hach 5B test kits is sort of bluish without a drop but turns to blue in a single drop, your hardness is approximately anywhere from 1-17 ppm. Anything below 2 grains is good for the vast majority of applications including residential and most commercial applications including carwashes, dish machines, ro pre-treatment, etc. Steam boilers, EDI pre treatment, and many other commercial/industrial applications require extremely low hardness, <5 ppm or 1/3 GPG is often required and we have test kits designed for these applications. For your use, the Hach 5B is perfectly adequate.