phosphoric acid

I brewed a Pilsner Urquell clone yesterday. I was doing an experiment to quantify the effects of starter vs. non-starter by splitting my batch into 2 fermentors. Anyway, I've been starting to measure and try to adjust for mash pH. So BeerSmith2 was telling me to add 5 ounces of my 10%...

Forgive me for asking if I have looked over it but I couldn't find any information on this here. I would like to make my own phosphoric acid solution as it's much cheaper then the stuff we can buy from the homebrew shop here. I found a seller that sells by the 1kg food grade phosphoric acid...

I use London tap water for brewing and have always had good success with this. I had a full water report done a while ago and regularly check the alkalinity and calcium hardness values before brewing. I use Bru'n'Water to adjust my water using 75% phosphoric acid and usually calcium sulphate...