Not necessarily a forum suggestion but a dern good one.
What I plan on doing is shock my pool and add a certain amount of trichlor tabs to a floater that way it will be around 3ppm chlorine when I get back like most other people do. There is already enough data for the variables to figure out what the chlorine may be, just need someone to write the code and do the math.
Information (variables that would be in the calculator):
Starting FC level
CYA level (needed to figure FC Loss per day)
Number of trichlor tabs
FC level loss per day (figures here: phosphates-and-sudden-chlorine-drop-t49199-20.html)
You could even throw in a TA and pH section and probably figure what your pH would be at the end. Maybe someone will volunteer to make one of these calculators. Maybe someone already suggested it, but I didnt see it anywhere.