A lot of people who are pretty qualified do it the method you outlined, filling the calmag, then filling the nutes to the "target" PPM, but it's pretty misunderstood and is definately not a universal method accross all nutrient lines.

1st, pour your self a gallon or two of your RO water and add CalMag until you reach 250-300. Then you know how much mLs it took and just use that amount each time, no need to play with the meter.
2nd. Use your nutrient manufacturer's recipe for the growth stage. Whatever amount they call for, use that.