Nute PPM Starting Question

I was curious on fellow aeroponics and hydro growers on this one.Do you guys deduct the original waters ppm from the ppm you want your nutes at? Meaning if Fox Farm say like week 5 ppm should be 1300 but your plain water started at 400 pm would you test for a ppm of 1700 or would you just come up 900 additional ppms from the original 400?Curious as I usuallysplit the difference to give some wiggle room so if my water is 400ppm without nutes and I wanna be at 1400 I add nutes till the actual meter says1600 ppm at finish.Just curious how others take this in to consideration..

subtract the ppm of the water from the goal ppm. in this case, if your water is 400ppm to start, you would add 900ppm.