It looks like any of the drop checker varieties will work as well as any other. The most effect the differences would have is in the response time, and that seemed, in my testing, to be about 2 hours no matter what the differences in design were.
The solution to use in the drop checker is water having nothing in it that affects either the pH or the KH other than carbonates/bicarbonates. The easiest way to do that is to use distilled or DI water and add sodium bicarbonate (baking soda) to get the KH you want, which is 4.0 if you want the solution to be green at 30 ppm of CO2 or 5 if you want it to be green at about 40 ppm, which is almost too high, in my opinion. The accurate way to get a known KH is to weigh dried bicarbonate of soda and add an accurately weighed amount to an accurately measured volume of distilled water to give ten times the KH you want, then mix an accurate amount of that water with 9X as much of distilled water, which will give the desired KH. For the amount of dried sodium bicarbonate to use, add 4.99 grams to 5 liters of distilled or DI water. This makes 40KH. Add 10mls of this to 90 mls of distilled or DI water to make a 4KH solution. Then add enough drops of pH reagent to get a strong blue color, but still transparent. It is best to do this only to very small quantities of the 4dKH water, not a big bottle of it.