Plant:

I've erased my answer twice here iin typing (groan) so am going to answer your q quickly before I erase it again and then I'll go back and address the rest.

I spoke to The Brain. He suggested that if you use tap water to cut your total numbers by about one third and then add the number of your tap water on after that.
If I were using a total of 600ppm (CalMag 300 and Pro Grow 300) and were using tap water I'd go with Pro Grow 400-500 plus whatever number your tap water is. He felt doubling the base nute numbers would just be too much.

The Brain also said that among different plants, strains etc he has seen variations of plants' ability to take nutes from 900-1800 depending on the plants so it's quite individual. Start on the lower side and work up keeping a close eye out for nuteburn.

I originally put all this info in one post that I then accidently erased EVERYTHING so I'm going to post this part and in the next post I'll tell you what to watch for as far as nuteburn.