Yep, same thing as when you make tea or coffee. It gets "spent" - and the rate at which this happens is entirely different depending on your water chemistry. There is no formula that I know of, the closest thing I can think of is using something called a Pearson Square:Apparently, I have to change the peat everytime I add new water to the buckets to be softened! (Nobody told me that!)
http://www.wetwebmedia.com/fwsubwebindex/fwh2oquality.htm
Peat is really quite unpredictable, it is why it has fallen out of favor vs. a reverse osmosis unit which gives you the same consistent results every time. In all of my pre 1980's aquarium books, the method you describe is mentioned for keeping "sensitive fish" such as discus (many of which nowadays, are acclimated to less extreme water chemistry)
I worry when you say things like "because my pH has to be around 7.3"...that you are missing the point of what I'm saying. Stability is more important than shooting for a precise target magical pH value.
I really think you should make an extract, so that you have a simple recipe to follow (boil X amount of peat in Y amount of water to make Z amount of extract) and use this in your water change water...much easier than the math equations you are posing

Google DIY blackwater extract for more info. You can also use leaves: http://www.tfhmagazine.com/details/articles/leaves-for-aquaria-full-article.htm
All of these measures = plenty of extra TLC for your fish...
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