Not considering that there is quite a lot of discussion about the usage of the acronyms MTBF, MTTF, MTRS and so on, this is simply wrong (or at least inconsistant):
MTBSI = Available time in hours / Number of breaks
MTBF = (Available time in hours - Total downtime in hours) / Number of breaks
Available time to my understanding is the time the service is up and running properly. Some people call it "uptime", though that is easily confused with the moment in time the service comes up. So it should be:
Available time in hours = uptime = Total time - downtime
Hence: