FWHM of something which does not reach half of maximum?
It depends what the reason is why the curve doesn't drop below 0.7.

If it's because of a background input at your sensor (a common situation in optical measurements), then you should subtract off that background value, and you'll end up with the formula you proposed. 

If the reason is something to do with the filter itself, then that isn't an appropriate solution. Maybe you could extrapolate curve to the half-maximum values, or maybe the concept of FWHM doesn't apply to this filter at all.