Same way you measure a 5 gal bucket, to the top. It's up to you what level you want to put in it. Depends on the size of the basket you use.. You can't depend on spoon feeding to get all the info you need, you gotta read.
What I am going with my RDWC system is mark the container and fill first time using a flow meter. As you fill, you can mark the container as you cross certain points. I attached a bottom fed water pump to a hose using fittings to convert one to the other, and attached the flowmeter to the end using a threaded to barbed fitting. Converting like 3/4'' threaded to 1'' barbed since I plan to use 1'' vinyl tubing for pretty much everything. This way I can just drop the bottom fed pump into my bathtub essentially and pump the water into the next room calculating it all. I wont actually use the tub, will have a bucket in there with the pump in it under the faucet just to avoid sucking in any residual soaps or whatever from the tub.
So in my example as I fill up via the res. once I hit certain bench marks ill mark the container. 10 gallons, 20 gallons, 30, 40, etc. keep filling until water touches the bottom of the buckets and then mark what that volume is. This way you will always know within a few gallons how much water is left in your system, how much has been used, and how much you need to fill.
I am not an expert grower by any means but being able to precisely identify how much water is in your system is critical and is going to dictate a lot of decisions, how much water to add, nutrients, etc.