Holy Crap Adam,
Grab a cappuccino.
The Block & Tackle will come with a certificate stating what load it is certified to safely use (SWL). The Breaking Load with be significantly higher. (Man Riding applications have a sfety factor of 8)
This information should also be stamped on the side of the cheek plates.
As long as the line is sized correctly, you will not have to worry about the line breaking under load. If the Breaking Load is exceeded, the Block & Tackle will fail before the line parts. Unless this is a 'shock' load, you will have plenty of warning before that happens.
Over simplifying it, every time the line passes over a set of pulley's, you will half the load. Hope I actually answered your question.