It is your boat and if you want a tank minder system, to feel safer, by all means, have one installed and calibrate against your sight glass reading. This may take a while..... depending on where you stated.
If you are lucky, you have rectangular tanks, you can figure out gallons per inch. Blue tape near the sight glass, magic marker, and you can at least figure the gallons per inch. Over time, you can calibrate the tank minder for your fuel tanks. Of course, this will be much easier if you have room in one tank to transfer fuel, one tank to another...... calibrate the sight glass. Fill one tank full, calibrate the tank minder to full then transfer fuel between tanks, calibrating the sight glass and the tank minder. Yea, I know, what I am saying is a bit foggy but think about it and it will all become clear.