I don't have a KH tester, but mh GH tester says to keep adding drops until it changes from yellow to green (shaking each time). Once it changes to green, count the drops and look at the GH chart for the different fish you have to make sure it's in the right range. On my kit you multiply the drops by 17.9 and get your Parts Per Millon (ppm) or percent GH in the tank. I would guess by the lst post that KH has a similar method. For example, it took mine 13 drops times 17.9 is roughly 230 ppm. This is actually rather high. Frequent water changes should fix this though eventually.
as far as accuracy, you are only going to be accurate to the nearest drop.