Just a thought on what Terry said about adjustable timing, you said you couldn't have a potentiometer as you lack an analog input pin, but I guess you could have an ordinary push switch as the input, then you could for example have it so you press and hold the switch for 2.5 seconds or 25s and that would correspond to the valve being open for 250ms, or have it so you press it multiple times with each click being an additional 10ms of open time. Obviously this is harder to make precision adjustments than with a pot but perhaps could be an option?