i use tap water with a ppm around the same as yours and i am currently doing dwc. i do not bother adding calmag and i am seeing no ill affect's from it. personally i think any time i do add calmag it puts brown spots on my leaves. when i don't use it with my tap water the spots stopped. the reason why i don't add calmag is because starting with like 180/200ppm dose not leave allot of room for nutrients before you start hitting high ppm range and start getting tip burn. its better to use your ph swing correctly at least that's what i am finding. this may be incorrect but its working good for me