use an app like growbuddy and take the information off your guarnateed analysis labels.. it should tabulate your total PPM and per nute PPM.
I'd wager you need 100 ppm or more Ca.. A good starting point, anyway. With RO water, you may need more. If you can't get there, that's just a failing of the fertilizer system/package etc.
Some other options to add Ca are triple superphosphate, but it's not 100% soluble, so you have to filter out some solids left behind. it's also extremely high P - 0-46-0 and 18 or 19% Ca, so it's only an option if P is on low side.. you can probably go up to 60-70ppm of P and never see a problem in the plant feeding it from a young age all they way till the end.
YMMV a bit on these numbers. if something is inordinately high in their formula, it may inhibit other things etc.. so the overall balance is just as important as any 1 component. there are many realtionships between all of the molecules.. mostly antagonistic, but some are stimulative too -- the difference beyween inhibiting something and needing to rais certain things together. (see mulder's chart)
Vege
N 120-130
P 40-50
K 180ish
Ca 100+
Mg 75ish
S 100+
That's with 300ppm hard water from tap that likely adds some Ca. I've also used soft water and still needed roughly the same formula. In flower (right at 12/12 flip) i drop N to ~100ppm, but try to maintain everything else. Could probably go a bit lower, too. I'm getting some dar leaves around 6-7 weeks into flower, so i need to go even lower or drop it sooner. The plant dictates... observe and react.