I have had spouts on many of my fresh tanks that tell when they are full. I just run and shut off the flow and the expanded tank spouts out until it equalizes. Sometimes I run a gallon or two down the drains to put some into the grey and black for slosh.
I have had DW watch the fill gauge too, and timed it after it gets to the third light out of four, but never had a problem filling to spout out.
I have had one rig that only filled from a pressure source too.. A mistake in design, but never caused a problem, not once.