Why does my mash temperature end up too low?

If you are transferring to another vessel, like a cooler, you need to add 4-5F to the strike temperature. This will compensate for heat lost from pouring.

Why did I end up with way too much pre-boil wort?

Most likely, you squeezed extra wort out of the bag. If you prefer to squeeze everything out, subtract about 1" (depending on the size of the grain bill) from the mash water volume.

Why is my mashout temperature not exactly 170F?

Mashout volume is difficult to calculate for BIAB so I calculate a boiling water mashout volume of 30% of the total mash water volume.. This will get you extremely close to 170F. Some batches will be a tad over, others will be a tad lower, but it should be within 1-2F (Typically over).

Are there any resources for Brew in a Bag?

You can read an excellent and FREE eBook on Brew in a Bag at: http://www.bestwaytobrew.com/

Where can I discuss this calculator?

Check out this thread on the HomeBrewTalk forum.