There is no clear cut answer on a lot of this. There are different kinds of mulch. Are you talking about wood chips, wood shavings, straw? Wet mulch may weight more than twice as much as dry mulch per cubic yard.
Different materials will weight different amounts, depending mostly what is the parent material. I found this in the internet just to give you an idea.
Sand 1.10 - 1.25 tons(2,200 - 2,500 lb.) per cubic yard
Planting Mix 1 ton (2,000 lb.) per cubic yard
Lawn Dressing .90 tons (1,800 lb.) per cubic yard
Compost .40 tons (800 lb.) per cubic yard
Landscape Gravels 1.20 - 1.35 tons (2,400 - 2,700 lb.) per cubic yard
Washed Gravel 1.35 tons (2,700 lb.) per cubic yard
Washed Limestone 1.20 tons (2,400 lb.) per cubic yard
Limestone Base 1.35 tons (2,700 lb.) per cubic yard
Lava Rock .50 - .65 tons (1,000 - 1,300 lbs.) per cubic yard
Learning your equipment and the sag is a real good idea. You might even paint a mark when you get to 1000 pounds if you have something you can mark. Oceangoing freighters do that, paint a "Plimsoll" mark on the vessel so when it sinks in the water so far they know it is loaded.