Measuring wort/water volume

Would a wooden dowel with notches be safe? Or what about using electrical tape? I've considered using PVC as well but was nervous about it nor being food grade.

I use an old keg spear, notched with tubing cutter at gallon intervals. Used an electric engraver to note how many gallons.

I used a notched wooden dowel. Works fine.

1/2" copper pipe notched with tubing cutter. Used a Sharpie to mark the gallons but it wipes off after a few uses so I have to reapply. I like the engraver idea. Might have to try that.

Your best bet is to calibrate and mark your fermenter. Use a 1 gallon bottle and use a measuring cup to mark quarts on it, then use the one gallon bottle to fill your carboy 1 gallon or less at a time. Mark on the outside of the fermenter with a Sharpie. With a carboy it's pretty straightforward, but it's still easy with a bucket - the plastic is translucent so you can see the liquid level if the light is bright (if your room light isn't bright enough, use a flashlight).

Either I had a moment of absolute genius, or total stupidity but I measure the pot, calculated the water level at different volumes, and used a aluminum yardstick to check the water level.