My system uses all 1/2″ interior diameter hose, so I just matched that for the clear hose.
To add graduated markings to the hose, I added 5 gallons at a time, marking the hose at 5 gallon increments. Rather than filling a 5 gallon bucket and attempting to funnel it into my fill-port, I timed how long it would take to fill the bucket with the water hose on full blast (45 seconds). I then added 45 seconds of water into the boat’s water tank. Close enough for government work. The tank on my Cal 39 was advertised as 50 gallons. I ended up adding 49 gallons using my method, so I must have been 1 gallon off.
Simple, cheap, effective.
Negative comments: The level indication moves around quite a bit while underway and varies depending on the tack I’m on. Averaging seems to be pretty accurate. It’s spot on in a flat calm, though.