You can unplug the pressure line that goes from the fuel tank to the muffler. When the tank is empty, fill it with your hand pump and count how many turns it takes until fuel starts coming out the pressure line. This will give you an idea of how much fuel to put in. You can do this after each flight and figure about how much fuel you use for each flight, then use that many cranks in the future.

I don't think fuel going into the muffler when you fill is that big of a deal, as long as you are paying attention and stop filling at the first sight of fuel. Whatever is in the muffler will blow out when you start.