Mash absorption

Is there a formula or rate that grains absorb initial strike water?

I would start with an absorbtion rate of .15 gallons/pound of grain to begin with. It all depends on your system and how you're crushing your grain though. .15 is average.

I have a total capacity of 5.5 gallon. I assume that if I start with 4-5 gallons strike water and add my 12 lbs of grain slowly while stirring them in, will I exceed my pot capacity?

My last batch I used an initial 3-4 gallons, and then sparged about 2 gallons for total of 5 gallons. This was a BIAB. I then also added 1 gallon to fermenter. Would this wash out my efficiency or ABV. Or am I better off just making 5 gallon batches until I can get a bigger pot?

http://www.rackers.org/calcs.shtml

If you are doing BIAB, squeeze the bag to get better efficiency and less grain absorption. I've seen .33 QUARTS/lb used as an estimate of absorption that couldn't be squeezed out. YMMV.

when it states in your calculator link that 12 lbs grain will take at 1.25 thickness 4.71 gallons of space.....that is total? Sorry if I'm asking a not so smart question.

Yep. Used this calculator last week for my first AG and it worked like a charm. For your 12lbs of grain, qts/lb of 1.25(average), TOTAL space used is 4.74gal making a 20qt(5gal) mash kettle/vessel ideal(minimal headspace equals better temp retention during mash)

Oh, seek out Deathbrewer's stickies for BIAB/AG BIAB in the Beginner section.

Yes

Will depend on grain weight if you BIAB. Since I almost never go over 5 lbs nor less than 3.5 lbs my loss seems to be only 0.0625 gallons/lb. With that small amount of grain I can really press most of the water out after the mash. I set up a spread sheet and kept track for 5 or 6 brews.