How much grain can you fit in a 10gal mash tun?

I have a ten gallon Rubbermaid beverage cooler with a false bottom as a mash tun. I know this is a pretty common setup so I want to see how much grain those of you with the same system can fit into it.

I do ten gallon batches and have never had a problem with too much grain to fit. However, the most I have ever put in was about 24lbs and if I can remember right I think it was pretty close to the top. I have a recipe I want to make that would feature a 27 or 28lb grain bill plus some rice hulls. Will this fit or will I not be able to fit enough water in there for the mash? I am alright with having to do an extra sparge, but would hate to be mashing in and realize I can't fit it all.

You can fit that much grain, but you will have a little thicker mash. You would defiantly have to do two sparges.
http://www.rackers.org/calcs.shtml

I can mash 25# in my 10g cooler. That gets me 10g of 1.060 beer.

I already do two sparges, I was saying I wouldn't mind having to do a third. It looks like if I do about 1 qt/lb ratio I can fit it without an issue, this eases my mind.

The greenbay rackers calculators are great....I use them for figuring my mash temps. But one thing to keep in mind about the "Can I Mash it?" is that it's not taking your false bottom into account. Dunno about yours, but I figure an extra .5 gallons (to air on the safe side) because of the empty space. Looks like with some attention, 28lbs is possible....just depends on the way you mash. I like doing step these days (and fly sparge), and with that method I found 21-22lbs got me right to the top of my 10g rubbermaid. Single mash, though, I'm sure you could fit more grain in.