It should be close to those numbers. However, you should get something to calculate these changes--like the Bru'n Water spreadsheet.
As for the stir, there is no problem with stirring during batch sparging--it is actually part of the process. I stir several times during the mash--usually every twenty minutes--then do batch sparging, where I fill with a calculated amount of sparge water, stir, let rest, vorlauf and then continue the lauter. Sometimes I use multiple sparges. I calculate these with BrewSmith.
Stirring will not draw tannins, because as was already said, this happens with pH and pressure. With stirring there is no increase in pressure--just agitation. As for squeezing the bag, I'm of the opinion that it would take far more pressure to draw out tannins than you could possibly provide with a little squeeze. Others disagree and are free to not squeeze their own bag. I promise not to squeeze anyone else's bag against their will--ever. Wait! That didn't sound right for some reason.