Drag a sled backwards for 8 rounds of 20 seconds on and 10 seconds rest. To gauge the weight correctly you should be able to get at least 50 feet per 20 seconds of work. If you're getting 200 feet it's way too light. If you're getting 20 feet it's too heavy. Adjust accordingly by doing a few test runs before you start the actual wod.
Post load and total distance dragged.