So, firstly, my calculation of Pag w/o halides is jsut mathematically incorrect, and secondly, it appears to be just this side of impossible to calculate those values, which is why everyone in here is after the holy grail of a sensor to do it.
Thank you all for the chemistry lesson, I'm afraid it's been a decade or so since I tried to figure out a pH, and a lot of organic -OH groups have been through the brain since then.
I guess my last follow up question would be - lacking a sensor and computer controlled additions, is it best to give up trying to calculate pAg in the kettle and just use a trial and error approach to get what you're after (in terms of grain size and shape)? is it really as difficult as it appears, or would a non-chemist like myself have a prayer in doing it?
OH, and PE - what do you mean the pAg is in reference to another salt, I figured the oncentration is the same, regardless of what else is in the solution (at a finite point in time, naturally it will change as things react, I guess, at time zero, when the Ag hits the stirring rod, does the pAg change even though the Silver Nitrate hasn't reacted yet?