Big Beer: What Water to Grain Ratio?

I am brewing a big beer this weekend, 1.084 or slightly higher based on ending efficiency. The grain bill is nearly 17 pounds.

For a beer of this size, what water to grain ratio is recommended?

The default Beersmith 2.x is 1.2 or 1.3 quarts per pound.

Yea, try to stick with at least 1.25qt/lb if not 1.5. You will have an efficiency drop with so much grain so plan accordingly. Some of that can be averted using more strike water but you run into problems with the MLT being too small.

I mash in a 10 gallon cooler so there should be plenty of room.

"Some of that can be averted using more strike water" - what do you mean?

I have found that on some of the big beers splitting the Mash actually gets you a higher overall effiency. It takes longer but it helps.
Also, I tend to go higher than the typical 1.25qt/lb.
I did a barley wine and used 1.75qt/lb for each mash and reduced the sparge amount. I ended up after 3 mashes (10 gallon batch) with a 1.098 OG, I then added a bit of Corn Sugar the 3rd day in to fermentation to bring it up to 1.102.

I'm making a beer with similar stats this weekend.. 18lbs grain for 5 gallon batch.. what's the ratio for sparge water? If I do 1.3 qt per lb of grain for the mash-in, should I use about 2 qts/per lb of grain for sparging?

I have started with a thicker mash...say 1.2q to hold at lower temp then raised the temp and thinned to around 1.5q just to get better extraction.