yes, it does.. so if you have tap-water with an EC of 0.6 and your plants are in a state to handle 1.0, then your nutrition will be only 0.4 ms..
my problem is that my tap-water has an EC of 0.8, so i had huge issues - especially with seedlings - before i got myself an RO filter (cost me 50 bucks and was the best money spent). now i start at 0.0 and bring it up to 0.4 with calmag before adding in other nutrients..