Install a tank level indicator.
And check it before flushing. If your tank is plastic, I recommend the Snake River Gauges...no sender inside the tank to clog and require removal and cleaning...instead, foil strips on opposing sides of the outside of the of the tank which measure the resistance between 'em. Check 'em out on the Snake River website at www.snake-river.org (don't ask me why/how they got a .org)...you can buy 'em from the online store here.