marking or keeping track of water in a stock tank

[reef tank 2.0]

for those of you that use / have used a rubbermaid stock tank for a sump.

my plans are to do all my water changes out of the stock tank sump, located in the basement. when it comes time to drain the water for a water change, how can you safely mark the water level inside the stock tank so you know how much exactly to drain / fill? what options are there to do this? with the tank baing black, it's hard to see any markings

 

[BeanAnimal]

A white PVC pipe with markings or even plastic yard stick.
stand alone or even the return pump or overflow standpipes with markings.