Batch Sparging - Newbie Question

Okay,

So I am now all set for allgrain brewing. I have a mash tun - a converted beverage cooler with a bazooka screen, and a nice big brew kettle with a ball valve tap on it. Best of all, its solid stainless and works on my induction hop inside. I have done two brews already, both using batch sparging methods, and I am using beersmith v2 as my brewing software. 

I just have a couple of questions as far as the sparging goes. I know to take the first runnings off until they run clear, and then recirculate them through the top of the grain bed. My main question is, do I just keep refilling the mash tun, giving it a stir, then running it into the kettle until I get the desired amount of wort pre-boil (and does anyone know a rough rull of thumb as to how much is lost due to evaporation during a 60 and a 90 minute boil). Or is there a set amount of sparge water that should be run through the grains, and once thats it, then thats it? Following the suggested amounts of sparge water in beersmith has left me a little short of the volume I was expecting by the time I had cooled it and added it to the fermentor.

Any advice would be gladly accepted!