Cal mag question about advanced nutrients

Hi I'm a pretty new grower on my 3rd now. I'm growing in soil, I've been using biobizz nutrients line which I've been happy with except the cal mag which I found had gone off after each grow in the down time, so I've bought advantage nutrients sensi cal mag extra.

Sensi instructions are 2ml per litre once a week. My question it that the region I live in tap water is very soft, it come out the tap at 45ppm. Do I still only use it once a week or should I add it with every watering?

what soil you useing? all soil will loose its nutrients and become a hydroponic medium basicly. unless you add organic amendments to it to keep it healthy.
try it at half that rate once a week and see how it go's. you can allways add more if it shows signs of a deficancy.
allways err on the side of caution, its easy to add more, not so easy to get rid of it once its in the soil