you should probably not plan on using one reservoir. different plants like different concentrations of nutrient.
a tomato plant in full grow or bloom cycle will easily eat up 1/2 gallon of water a day when its hot out. how many linear feet of pipe are you using? if its Ebb n Flo, multiply the area of the pipe times the length. divide by 144 to get cubic feet then multiply by 7.5. This will give you the number of gallons required to fill the pipes. plus you'll want some excess, so maybe multiply that by 1.5. using the general hydro flora series, you'll use approx. 6 tsp per gallon(total of the three). there are 48 tsp in one cup.
bear in mind that an actively growing plant will transpire a lot of water , but not as much nutrient. at the end of the day you'll want to bump you water levels up to the EC the solution started at to account for this water loss.
what kind of system are you growing?
besides tomatoes and broccoli, what plants are you wanting to grow?
do you have an EC meter and pH meter?
Re: hard water, I can't help as mine is not. I'm pretty sure others around here do it though.

Hard water means there's dissolved calcium and magnesium in it. Two things that you want for your plants anyway.

My plans would be something like this made into ebb&flow
http://www.jasons-indoor-guide-to-organic-and-hydroponics-gardening.com/homemade-aeroponics-system.html
My pvc would be 1/2" 24' pvc inside 6" 24' pvc that would flood then drain slowly back into the reservoir. Other plants would be cauliflower, cabbage, and hot peppers.
I haven't purchased any meters just yet, but was aiming for the ones in the 40$ range.

well broccoli cabbage and cauliflower are all in the same family as tomato and pepper are also. however tomatoes love nitrogen while peppers do not. so you're already up to 2 maybe 3 different reservoirs. best to focus on one crop and stick the rest in dirt. you can always use your nutrient waste to help them along.
I'm not sure why you're run the 1/2" dia inside the 6" dia. that's just more places for it to leak.run the supply lines outside and have it deposit the nutes in the top of the 6"dia at the top. have the drain and overflow coming out the bottom at the opposite end.
if you can get a truncheon meter for $40 then get them both. otherwise, just use litmus paper for pH, take the $80 and save up a little more to buy the truncheon. Especially with hard water, knowing your EC will be important.