If this is about sight glass levels, read on:
You can have bottom drains. But regardless, you calibrate you sight glass in a brewday scenario. First you put everything in your pot that will be in there on brew day, such as false bottoms, hopstoppers, dip tubes, etc etc. If you pour a gallon of water in and it's not shown on your sight glass, you pour another and another until you get your first marker on your sight glass. If it took you three gallons to get your first level indicated on the sight glass then your sight glass will start at 3. There's a reason BobbyM starts his decals off at the number 2. A sight glass that can indicate one gallon is extremely rare because of the diameter of the bulk head and the 90* fitting height.
So typically once your HLT gets below 2 gallons it becomes very difficult to accurately measure how much water volume you're draining. That's why most people put more water in their HLT than they know they will require. Most people refill their HLT after mash so they know they'll have more than enough for sparge and to get the BK to their boil volume.
Hope this helps and I hope I'm not too drunk to know what I'm talking about.