WLL is a calculated fraction of breaking strength, so consider that WLL provides the necessary safety factor by default. Your snatch block must have a capacity greater than the weight of the load hanging from the hook, since the snatch block carries the full weight of the load. Likewise, the winch line rated working load, or single line pull, already has the safety factor figured in. ANSI 10.31 requires digger derrick winch line to have a safety factor of 5 to 1. Just ensure the winch line pull is not exceeded and your safety factor is intact. When 2-parted, each part of the winch line above the snatch block will carry 1/2 (half) the total load weight. Check the capacity of each component of the lifting system, including the rigging, so you don't overload any individual component. When lifting at maximum capacity, remember that all capacity values are for static loads. This includes the capacities listed on the load chart. Rough or abrupt movement can dramatically increase the forces on the lifting system and can exceed capacity. This could result in loss of unit stability or structural damage, which puts everyone in danger.