I assume you are estimating cya at 46 since you are filling up between the 50 and the 40 line? You can safely swim up to your SLAM level for your cya level...so for 46 ppm cya, you can swim up to 18 ppm FC.
Also, from you FC values, I am assuming you are using the 25 ml sample size? Go ahead and use the 10 ml sample size instead, since you don't need better than 0.5 ppm precision and you will save reagent (and a little time).
Also, from you FC values, I am assuming you are using the 25 ml sample size? Go ahead and use the 10 ml sample size instead, since you don't need better than 0.5 ppm precision and you will save reagent (and a little time).