BIAB volumes

I seem to be stuck in a rut on water volumes! I plan to start doing all-grain BIAB brews, but my preference is to sometimes do 2.5 or 3 gallon batches or 5 gallon batches. I need some help in determining my water volumes. I have both a 5-gallon stainless kettle and 9-gallon stainless kettle. I use a standard turkey fryer to boil, assume 60 minutes boils. Since I plan to use the BIAB method, I want to make sure I get my initial water volume correct.

Questions:

1) How much should I assume I boil off in 60 minutes for 2.5 or 3 gallon batches and 5 gallon?
2) How much should I assume for grain absorption?
3) How much should I assume for lost to trub for 2.5 or 3 gallon batches and 5 gallon?

Thanks a lot for any help!

I'm exclusive BIAB.

1. Measure it. I get 2 gallons an hour.
2. I'm dialing this one in. I squeeze the heck out of my bag and get .05# per gallon absorption on around 10# of grain. I have a feeling this number is influenced by the amount of grain but haven't proven it yet.
3. I use a hop sock and I dump it all in the fermenter so I can't help you there.

1) Like Daddymem said measure it. I evap 1.5 gallons in 75 minute boil
2) Good rule of thumb is .1 gallon which is what I get. So roughly 1 gallon on a "normal gravity" beer. I just clamp my bag to the side of the keggle and let it drain.
3) I use a hop sock also and end up with very little trub in the kettle. I don't account for fermenter loss, but probably should. I usually have well over 5 gallons of beer in the end.

So my calculations go something like this. I usually just figure 1 gallon for grain absorbtion. I know I boil off 1.5 gallons in 75 minute boil. For a total of 2.5 gallons of water over the quantity of beer I get. I start with 8 gallons of water. I calculate my recipes for 5.5 gallons.

I boil off a gallon per hour and always do 90minute boils.

I do 10 gallon batches, and loose a gallon between break, trub, etc, so I aim for 11 gallons at end of boil.

So I start with 12.5 gallons at start of boil. I average .1 gallons per pound absorption, so it's pretty simple math to figure the amount of "strike" water:
12.5 + .1 * pounds of grain

When I do a partial mash, I generally just aim for 3.25 quarts per pound as long as it will fit into my pot. (I wrote a little script that figures it out for me.) I do this a lot as I bought a 15 gallon kettle rather than a 20, like I should have. But a few pounds of DME is no big deal... So, make sure your pot can handle the volume if you feel dirty for using a little extract