Hang onto your confidence and let's work through what is most important in this process. There are many good folks here who regularly use these kits and can help you figure out why you experienced differences like this from the same sample.
Is this a new kit? Are all reagents dates current? Vials cleaned before and after testing?
Now TFP recommends the 10ml sample test for FC for several good reasons. With the 10 ml test each drop used to achieve the color change from pink to clear equals 0.5ppm. Fewer drops, shorter test and a simple conversion (# of drops X 0.5 = FCppm). If you are in a SLAM use the 10ml sample and one big scoop of the powder. Using a SpeedStir, lighted magnetic stirrer, is a solid tool for titrations like this. Take notes and rely on the data to make your decisions moving forward.
The 25 ml sample test does provide a higher resolution but uses more reagent, takes more time and requires a different conversion calculation. There is really no benefit to running a 25ml test for a SLAM or regular FC testing. A resolution beyond 0.5ppm is not required.
Focus on your SLAM now. Your testing discrepancy may be as simple as an incorrect drop count and/or conversion calculation. It also may require more investigation into other areas such as your testing methods, the actual drop sizes, plugged or damaged applicator tips, etc.
