If using the R-0871 you would use a ratio of 2.25 x FC result to get your bromine concentration. This is simply because the same reaction is taking place but the molecular weight of bromine is about 2.25 times greater than chlorine. So... if you counted 19 drops of R-0871, that would be 19 x 0.5 x 2.25 = 21.375 bromine. With your R-0872, 15 drops would be 18.75 bromine based on the instructions. This is a significant difference in results.
The amount of powder is not critical, as long as their isn't too little. You essentially are saturating the solution with the R-0870 powder. Too much is not bad, too little is. Two level scoops or one heaped scoop is plenty, accuracy is not all that important as long as there is enough. An indicator that you know you have enough R-0870 is that there are a few crystals, particles of the powder floating in the solution as you have started to swirl/mix the solution. I have found that using a Speed Stir gives me more consistent results from test to test over hand swirling.
What I would do is contact Taylor directly and ask them what the difference is between the R-0871 and R-0872. Then you can address the problem as a difference in the two solutions or an issue with testing consistency.