Here are a few basic steps when it comes to planning your rigging and how the capacity of the sling can be reduced based on how you are rigging your slings.
Remember as a rule of thumb when using any two slings as a choker hitch, to keep the rigging angles about 60 degrees or better to reduce the chance of your slings sliding during the lift. This will also keep the capacity of your slings higher as the lower the sling angles are, the lower your sling capacity becomes. If you are concerned about the slings sliding, try double choking your slings. Try using shackles when choking nylon slings to reduce the friction, or wear and tear of the sling.