As mentioned by Solar Mike, you'll need to calculate the deflection at midspan.
One's first instinct would be to make use of the fact that joist structures tend to be heavily standardized and there are handbooks around which give you all the information you need (I personally don't have any because I'm from Brazil, which doesn't have a tradition of joists).
However, this first instinct won't do you much good. As standardized as joists should be, reality isn't as kind. The problem isn't really the interaction with the flooring you've mentioned, but the simple fact that wood is a highly variable material, with significantly different Elastic Modulus for every specimen of the same species. Given that the Elastic Modulus is a key variable in calculating deflection, that makes the calculation very imprecise.
Then there's the fact that buildings aren't always built exactly to code and construction imperfections exist everywhere. And calculations are always made in a world of theory, where supports are perfectly pinned or fixed, when reality is much messier.
So if you really need to calculate the deflection, you'll need to take a core sample of the joist (even though removing a beam sample is perfectly fine, good luck telling the owner that!) and do a pretty detailed survey of the structural system. And you'll probably still be significantly wrong.
The easiest solution here is to forget about actually calculating the deflection, and just use the strain gauge results directly.
Since you've apparently already bought some, stick one to your house and then see what the strain gauge measures when you walk around the room, when your dog walks around, when you move furniture around, etc.
See if you can distinguish that signal. If you can, then that strain gauge is sufficient (feel free to buy cheaper ones and try them). If you can't, then you'll need to buy a more sensitive one.
Obviously this will require significant calibration, but you should probably get used to that anyways, since you'll probably need to calibrate the results for each of your customers' locations.
You'll then have to figure out how to handle moving furniture (which will alter the "empty room baseline") and long-term effects like creep.