I suppose going with 7 segment display for now is good. I second you.
Concerning measuring the volume of milk using time of valve opening - it is a nice idea, i'll however add another variable - height of milk in the tank (we'll get from the level meter)
HERE IS THE EQUATION OF RATE OF FLOW OF A LIQUID FROM A TANK UNDER INFLUENCE OF GRAVITY
ALL OTHER FACTORS ARE CONSTANT EXCEPT HEIGHT OF FLUID & TIME.
FLUID VOLUME = RATE * TIME TAKEN
This computation can be done by the micro-controller, to accurately measure the output by varying time of opening valve.