You are correct. Factory installed sensors are just that, incremental. And don't count on them being equal in 1/4th increments though. The best you can do is use them for a relative range. Think of the "empty" reading as the red line on your gas gauge in your car. After the red line, you may still have 2 or 3 usable gallons left. In a camper, you may have 5 - 10 - 15, depending on the size of the tank.
After a while, your sensors will probably quit working, and that happens much sooner than later in the life of most RVers. Same is true with the sensors in your black and grey tank.
Filling a fresh water tank is no problem. Fill it till it over flows the fill port. You know it's full. But monitoring the usage just takes some time to as you get to know your camper, and that just take time spent with the camper. After a while, you'll not use those read-outs at all.
You'll learn the limits of your black and grey tanks too after a while.