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.
It sounds like you are using AP's KH/GH test kit. It measures in degrees. The Hagen KH/GH test kit uses a different indicator solution. That's why the colors are different and one measures in ppm and the other measures in degrees.
If you round 17.86 degrees up to one decimal place, you'll have 17.9 degrees.
You can have two ways to calculate ppm and degrees. If you take 1 and divide it by 17.86 you get 0.056. Also, if you take 1 and divide it by 0.056 you get 17.86.
Then, 17.86 ppm per degree and 0.056 degrees per ppm are your constants.
So, let's says you have 5 degrees of hardness. You can multiply 5 degrees by 17.86 and get 89.3 ppm.
Here's the other way to do it. Let's use 5 degrees of hardness again. You can divide 5 degrees by 0.056 and also get 89.3 ppm.
Let's deal in ppm this time. Lets say that you have 100 ppm. You can divide 100 ppm by 17.86 and you get 5.6 degrees. The other way is to multiply 100 ppm by 0.056 and you get 5.6 degrees.
Do you understand what I did in these calculations?
Here's a tip. Both of these test kits use 5 ml of water for the test. You can be a little more accurate if you can find a test tube that will hold 10 ml of water. When you get the end color, you divide the number of drops by two. This may help if you have trouble deciding where the end point actually is. I believe that Tetra and Red Sea use 10 ml test tubes in some of their kits. (You can also use 15 ml, 20 ml, etc. also. Just be sure to divide by the number of drops correctly.)
LaMotte's alkalinity test kit is more precise and it's about $20. It's a good one though. It's color change end point is rather abrupt. That helps it to be more accurate. LaMotte KH/Alkalinity Test Kit 4491- DR / Direct Reading Titrator Method - Marine Depot - Marine and Reef Aquarium Super Store