really the only thing that should change is your denominator; the numerator will stay the same if zeroes are added in.
So it should be
[code]
total(case when [date] in_range ?Date1? then [measure] else 0 end)
/
total(case when ([date] in_range ?Date1?)...
So it should be
[code]
total(case when [date] in_range ?Date1? then [measure] else 0 end)
/
total(case when ([date] in_range ?Date1?)...