I think it is important to understand resolution - these drops are meant to be applied to 5 ml of water and represent for 1 dKH/dGH for each drop.
A color change after 3 drops means your water is somewhere between 2 and 3 dKH/dGH.
If you want higher resolution, double the water to 10 ml, and each drop counts as .5 dKH/dGH. In this example, 5 drops would mean somewhere between 2 and 2.5 dKH/dGH.
Hopefully that gives some ideas on how to get a more accurate reading if needed.