Adding water to boil to lower temperature?

Newbie question:
I have a 5 gallon pot, so I have to do a boil of 3 gallons, I'm thinking, so I'll have head room to prevent boil over. That means I'll have to add water to the fermenter before I pour the wort into it.

Will 2 gallons of cold water be enough to bring the temperature of the wort down to 75 degrees or so?

I have a wort chiller but I don't want to lower the temperature too much, right?

Use your wort chiller to chill the wort into the 60s or 70s, then top off with water. Ideally you want to chill your wort to or below fermentation temps.

I don't think that will lower it enough. I'm not sure what temp your ground water is, but mine comes out near 70. when i brew, i'll sanitize a 1g pitcher before i start, and put it in the fridge. I'll use a gallon (ish) of tap, and that chilled gallon to top off after my wort chiller, and that usually gets my temps down to about 65-70.

8th Grade Math:

If you have 3 gallons of water at 212, and mix it with 2 gallons of cold tap water at say 59 degrees (I measured mine at 59 in St. Petersburg, FL), then mix the crap out of it, then you end up with 5 gallons at a net temp of 150.25. Too hot.

To get the net temp down to pitching temp of 80, you would need to chill the 3 gallons of boiling water down to 151.5 before you added the 2 gallons of 59 degree water to end up with a net 5 gallons at 80.

Thanks, guys! I think I'll store my water in my fermentation fridge at 75 degrees, then use the wort chiller to bring the wort down to 75 degrees and then rack on top of the water. Seems like a simple solution but it took a while for it to occur to me :-/

My fermentation temp needs to be 69 degrees. Is 75 a good pitching temp?

I always pitch at or below fermentation temps. If you pitch at 75*, those yeast could kick off before you drop to 69*. Yeast activity can raise the temps itself. I'd pitch at ferment temps and throw it in your fermentation chamber set to the right temp.

Thanks. 69 degrees it is.