Diluting feed to desired EC level C1V1 = C2V2

So I follow the GH Flora series feed schedule, using their recommended formula and then diluting it to my desired EC level for feeding. However, I'd like to eventually set up an automatic feed system using a reservoir. To save time when creating bulk supply, instead of having to dilute the the feed solution to my desired level, I'd like to be know how much water to add to a concentration to bring to my desired EC level to take out any guesswork and wasted time. I found this formula for diluting solutions online C1V1 = C2V2. The formula is easy but what if I'm diluting with calmag water that already has an EC of 600 to 800? How can I factor this into my formula? Basically, if I know the EC of my concentrate, say 2500, and I'm diluting it with 600 EC calmag water, how much of each solution would I need to reach an EC of 1400? Btw for coco I have read that I should always dilute the concentrate with calmag water in order to keep the correct dosage of calmag. Thanks in advance!

C1V1 = C2V2 seems inadequate since you have a third constant.
What do you think of this?
C1V1 + C2V2 = C3(V1+V2)
I could be wrong,,,,