I use Atami CAL&MAG, 0,25ml/L - 0,50ml/L ,
about nutrients im using "LUCAS FORMULA" using FLORA MICRO SOFTWATER + FLORA BLOOM.
don't forget im running aeroponics, my doses maybe not similar to yours. week 1-2 veg i use 0.5ml/l OF MICRO AND 1ml/l BLOOM, 2-4 veg i use 1ml/l OF MICRO AND 2ml/l BLOOM + 0,25ml/l CAL&MAG from atami (caution that this cal&mag makes ph drop, keep eye on that)... then normally i start flowering with 1ml micro 2 ml bloom, then i raise a little bit cal mag first 3 weeks to 0,50ml/l, then i play arround 1.5-2/ml micro and 3-4ml bloom...
That's how it works with me...
Now why regular meters wont read properly your water with a low PPM? Because low ppm water has low ions and this will include all carbonate alkalinity, for this reason, low ppm water won't carry any buffer system, meaning that a small amount of ambient atmospheric CO2 will mix with the water and result in a slightly depressed Ph reading as CO2 is an acidic gas.
With that said you must remember though that as the soft water is ionically low and the Ph registered doesn’t reflect the projected Ph of the water once it is mixed with a solution that is Acid or Alkaline. Maybe some more expensive tools can read this kind of water, but not our regular cheap meters, i have a good meter aswell (HANNA HI98130) , but not enough for this task for sure, the only thing i read from my soft water is the PPM.