Tap water ppm

My ppms for my tap water is around 120...should I add or subtract this number to get my desired ppms for feeding? Say I'm looking to get 400. Am I really at 280 or should I mix until I get 520?

520, normally tap water that low doesn't really have anything the plants can use.

Tap PPM counts toward your total so you would add the 280 to your 120 to get 400.

Lol,somewhere in the middle ha.After growing with the same water for awhile you'll get a feel for it.