I built my own brewing pots out of some 15 gallon kegs a few years back.  I think I was drinking when I built them because the sight glasses are at different levels making it tough to remember how much liquid is in each tank.   So every now and again I will “measure” and mark it off on the kettle which eventually washes/burns off.    So now for my boil kettle I am posting it here on my blog where I can find it:

Keg A:  Boil Kettle

• One inch below the middle weld line on the keg is 7 gallons.   My most common pre-boil volume.
• 6 gallons is just at the top of the bottom bulge on the keg.
• 5 gallons is just at the bottom of the bottom bulge on the keg.
• 4 gallons is inside the brass nut on the sight glass.