Wouldn't be out of a job 'cos you'd still have to scrub the windows and rinse. Wouldn't need to tell them your water wasn't quite so magical.
I think my tds would be too high to work - who knows. My shower doors get rinsed with 300tds when I've Viakalled them - don't seem to spot for a week or two. Someone tried Demon Shine on one of the forums but it left a wax coating. I remember one numpty was going to try white spirit on here years ago.
GG4 is supposed to contain something that dissolves calcium and magnesium deposits from hard water spotting - not sure if it still would at such low dilution rates.