What it means by premixing is don't mix the Gro, Micro, and Bloom together, THEN add that mixture to water. As long as you're adding each solution into your water one at a time you're fine.
Do not use a separate gallon for each solution; the easiest way is to add the nutes to a single gallon jug of water, then adding that water to your reservoir as needed. Make sure you store your nute solution in a dark, cool, dry place if you plan on storing it so no critters grow in it.
If you refill your reservoir weekly and are following the Flora instructions, it isn't super critical to have a PPM reader, but it is nice. What I would say is more important is a pH meter and maintaining the 5.5-6.5 range. Don't use the drop/color kits, get a digital that goes to at least 0.1 decimal places. If your ppm's are whack, all you gotta do is dump the reservoir and refill with fresh nutes to fix it, while maybe reducing nute concentration. If your pH gets too far out of that ideal range your nutes will lock out. I grow out of a home-built DWC setup using only a pH meter and digital temp/humidity meter. I have not had any problems not knowing ppm's thanks to the calculator below.
IF YOU'RE USING GH I HIGHLY RECOMMEND YOU CHECK OUT THEIR CALCULATOR
http://www.genhydro.com/calculator/index.html
You set your nutes (Flora Series), System Type (recirculating), Water type, Program level (I'd go with simple,) and feeding schedule (if you choose aggressive, use those numbers as MAXES or reduce nutes to normal schedule if any nute burn arises.)
After that set the size of your reservoir in gallons/liters, and if you want your results in ounces/millileters.
This calculator is AWESOME, it even gives you a shopping list to know how much of each product you need for your grow. When you get the results you'll want to notice a few things:
Growth Stage: This shows the phases of your grow from seedling to ripening. The numbers are arranged in columns above and below the stage they correspond to.
Nutrient Solution: This shows the ppm you want to aim for during the different stages. IF you have a ppm reader
Product Needed Per Reservoir Change in mL/oz: The info you want. This tells you how much of each product to put into your
ENTIRE RESERVOIR,
NOT PER GALLON. Example: I have a 2 gallon reservoir, and the calculator says to add 5 mL of each product. If I'm going to make a batch of nutes and I want to have leftover to add as needed, I have to divide product/reservoir size. So, if I add 2 gallons of water into my reservoir, I COULD add 5ml of each to the reservoir. OR, if I wanted to make extra to store in gallon jugs I'd add 2.5 mL to each gallon of water. I'm sure it makes sense I'm just getting long winded after a nice smoke
Reservoir Changes: At the bottom is the number of reservoir changes, but it's interchangeable with WEEKS, since you should be changing your reservoir weekly. Adjust these numbers to fit your specific grow style and strains so that your grow/veg cycles are the proper number of weeks long. These numbers are useful to give you a general idea of when to expect a harvest, but they're mainly there for the shopping list; so the calculator knows how much total product you'll need for the entire grow. Changing these has no effect on your product/gallon. I like to put dates below each one so I at least know where my plants SHOULD be performing. Like right now I know that I should be an expectant harvest daddy on April 20, 420 isn't that awesome! In all likelihood I'll probably be within 1-2 weeks of that date, but still useful when planning a grow.
I hope this helped! I just started my first DWC grow and I'm using GH nutes so if you have any questions feel free to hit up my journal. Cheers!
https://www.rollitup.org/dwc-bubbleponics/407954-first-dwc-grow-pics.html
