Strips will only give you a ballpark value - 0, 15, 30, 50, 80ppm is typical. They are not very accurate and they can bleach out easily with high chlorine.
The drop test above is similar to what is described here on TFP but keep in mind that the manufacturer has designed their test around industrial/consumer water sources that DO NOT contain a strong oxidizer like chlorine. So you may have problems with their test method if you don’t first neutralize the chlorine in your sample water.