RO has fewer unknowns, therefore easier to work with -- if convenient. This is the better way to go. sounds like you have a baseline doing it this way, so you'd be able to compare any deviations if you try tap water. that's a good thing. I doubt your eye will discern a difference as long as formula is adjusted properly, if necessary.
if terribly inconvenient or in a rush, tap is fine too. You'll have some extra Mg, Ca, S in there, maybe.. you can check your local water quality report for analysis -- in the usa and somewhre with "city" water system, vs a well. with a well, you'd need your own analysis on your dime, if you wanted it.
REally shouldn't make a huge difference. may need to adjust fomrula a few percent here or there to account for anything extra in tap vs RO.
Your water us quite hard. I trust your EC readings more than ppm readings as that is jsut 1 of 3 conversion factors to convert ec to ppm on those devices, and that depends on the manufacturer. My tap is ~0.6EC (same as 600 on the scale you used). I've not had issues with my hard water before. It's supposedly borderline at this level of concentration. YMMV.
RO definitely wastes more water.
if you switch, maybe drop your dose 5% and see what happens. i doubt it makes much of a difference. may end up jumping back up to original dose. of the "400ppm" it isn't a ton of plant nutes.. 10-20ppm of one thing might be the most of any 1. Ca, i'd wager, and usually can afford more of that.