A formula would work if the rates where somewhat consistent, but there are too many variables like time,temperature, available nectar, etc that affect the amount of travel regardless of the hive strength.
I have often thought about building an electronic counter, but it is on the wishlist which I never seem to have time for.
My thought was to use small screen cones (like the ones on some pollen trap exits) have the cone lead into a 1 bee wide tube. Place the tube between 2 IR sensors, so when a bee passes, it trips a counter. Since it is only one bee at a time, multiple tubes would be needed both ways so that you can count coming and going. A good PIC project, if I only had the time