Noob question - Hop Head Clone

OK, so I’m about to do my first AG and my OCD has kicked in.* I’m looking at doing an Imperial IPA (Hop-Head, Bend Brewing Company).* I put the information into BeerSmith which I just bought and I really don’t know if I’m inputting the info correct.* I’m using a 50qt Coleman Extreme as my MLT.* I’m using a Bayou SP-10 burner w/ 40qt stockpot, so I figure my evaporation rate will be around 15%
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5gal batch
15 lb 2-row pale
.5 lb Crystal 30
1.25 oz Saaz (1st Hop 90 min)
1.5 oz Chinook (90 min)
2 oz Northern Brewer (5 min)
1.5 oz Cascade (5min after knockout)
1.5 oz Cascade (Dry Hop)
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OG= 1.073* FG=1.017
Wyeast 1968 London ESB (Should I double pitch?)
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BeerSmith said to mash with 5 gal (155 60min) then batch sparge with 3 gal.* This may be my stupid question, but shouldn’t I sparge with more water?* Any suggestions will be greatly appreciated.
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I like to have more hot water at the ready than necessary just in case any thing happens. Just continue to sparge untill you have your preboil amount in the kettle. Pay attention to how much water you use, how long it takes and your extraction/efficiency rate. Make sure to write everything down so you can eventually zero in your brewhouse.

According to beersmith (using the water needed calculator) the grain will absorb app. 1.86gal, so you will need:
Water into BK: = 5*1.15 = ~6gals
Wort drained from mash = 5-1.86 = 3.14gals
Sparge Water needed= 6 - 3.14 = 2.86gal
3gals seam realistic....

Are you batch or fly sparging? If you batch sparge I would split the 3 gals in two 1.5gal cycles. I highly recommend Bobby_M's NMODBS method found here (essential AG beginners reading) :
http://www.suebob.com/brew/Bobby_Mallgrainprimer.pdf

You will likely need close to 6.5-7g gals of pre-boil wort if you are doing a 90m full boil.

Thanks for the replies. I'm going to batch sparge, I might do 4gal and split into 2gal cycle.

Just be carefull not to make the mash to thick (ie. go below the 1.1 qt/lbs) or to end up with too much wort after sparging (nobody wants to do a 180min boil ;-)

Good luck on your first AG brew!