Formula is correct but your calculations are off. You're probably rounding off too soon or something. Risk based on Samsung's AFR for consumer workload is 0.3197% for 2 drives and 0.6385% for 4 drives.
Because playing with numbers is fun...
MTBF for Samsung SSDs (830, 840, 840 PRO): 1,500,000 hours
AFR (consumer): 0.16%
AFR (enterprise): 0.58%
After some back calculations, it appears Samsung defines consumer workloads as 2,400 hours of operation per year. Enterprise is, as expected, based on 24/7 operation or 8,760 hours per year. Apparently, I should be using the enterprise AFR given my usage.
Interestingly enough, WD doesn't publish MTBF numbers anymore for the Green, Blue and Black. Lowest MTBF I've seen from WD, however, is 800,000 hours for the Se. Red is at 1,000,000 hours.
MTBF: 800,000 hours
AFR (consumer): 0.30%
AFR (enterprise): 1.09%
MTBF: 1,000,000 hours
AFR (consumer): 0.24%
AFR (enterprise): 0.87%
Edited by rui-no-onna - 8/26/13 at 6:18pm