The Tank Tender is simple and its accuracy seems quite repeatable. The tube doesn't do well in black water tanks (what does?), but for water and fuel
it's almost as good as a dip stick.
If you've got an unusual shaped tank, the best way to calibrate the unit is to put in premeasured amounts of liquid and record the gauge reading. Keep adding premeasured amounts until the tank's full, do a hard copy and post it next to the gauge.
Tank Tender also makes electronic versions that come with a 10 LED level display, powered pump, and a few other tweaks. These work well too.