Mash Tun Volume

Question about mash tun volume: I'm brewing an imperial stout tomorrow and will be using more grain (23.5lb) than ever before so I need to make sure I have enough room to mash. I have a 48 quart rectangular cooler and will be using 31 quarts of strike water. I just did a test with grain volume and weight (2 row) and found that 4 quarts of grain weighs exactly 3.5 pounds. SO extrapolating the math brings me to 27 quarts of grain which added with the water volume is 58 quarts total, more than my mash tun. Anyone know if the grain immediately absorbs water and therefore I will be ok? I was using the site below and it comes up with a very different result. I'm thinking it's assuming grain absorbs water immediately and is therefore using a different ratio on grain volume per pound. Thanks!

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

That's a good site and I find it to be pretty accurate. I can max out my 10 gallon tun at about 20# so with a 12 gallon you should be ok at 23# assuming you are doing a single infusion mash

Or just mash a little thicker than usual.

Thanks for the replies. I had enough room with a little to spare. The rackers site is right on.