I always use fully saturated brine (which is 35%, or about 35 grams of salt per 100 ml). I add the same amount of calcium chloride as I would for milk of the same volume and I try to make the pH around 5.3 or so by adding citric acid (probably about 1 gram per 100ml, but I can't quite remember). Over use, the brine will suck whey from the cheeses and it will stabilise at about the same level as the whey. Probably you can just start with whey (*slightly* concerned about the lactose levels, but my the same logic as above, I'll be getting lactose in my brine anyway). I store my brine in the cheese cave. I've had light brine (3%) go off on me, but never fully saturated brine -- I think it is highly unlikely that *anything* can live in fully saturated brine.
There is no need to make brine more than once in your life, as far as I can tell (2 years and counting for me). The only thing that is important is to add enough salt to the brine every time you use it to make up for the salt that goes into the cheese. It's pretty easy to calculate. If you want a target of about 2% of the cheese by weight, then you need to add 2 % of the cheese's weight in salt each time. For a 500g cheese, that means 10g of salt. Normally, I'll split that in 2, float the cheese in the whey, sprinkle some salt from one half on top of the cheese, and dump what's left in the brine. Do the same after flipping it half way through.
There is also no need to make a huge amount of brine. I use about 400 ml of brine for my 500g cheeses. I use a 1.2 liter box, place the cheese in it and then add enough brine so the cheese floats. That's it. 400 ml of brine is 150g of salt and only 10 grams is going into the cheese, so there is *way* more than enough not to worry about not having enough salt :-) Plus I'm adding the other 10g in there anyway.
Biggest pet peeve when looking at Gavin Webber's recipes: "18% fully saturated brine"... Which is it??? Then if you go to his blog post about making brine it's 450g salt for 2 liters of water -- so 22.5% ...
But then I realise: 22.5 / 122.5 = 0.18. So he's taking the percentage of the salt to the final weight, not the percentage of the salt to the weight of water.... So confusing...