pH calculations using multiple acids

[Saline]

I am trying to formulate to pH of between 3.0 and 3.4.  The slurry starts at about 5.5.  I have to add vitamin c as ascorbic acid to the slurry.  I would like to use a combination citric acid and ascorbic acid to get the slurry down to my desired range and achieve my label claim for vitamin C.  Can someone point me in a direction I can start researching?

[hypervalent_iodine]

Would it not be simpler to try different combinations, measure the resulting pH of the slurry, and adjust as needed? 

[Saline]

It would, but I'm formulating a pectin gummy that needs at least 3.6.  The problem will be as I add the acid to the slurry it will start to set at about 4.2, which is way to high for the optimal pH.  It will set up before I get to my 3.6.  If there is no quick way to estimate it, I will probably do it in DI water and try to extrapolate.