For fly sparging, I use 1.5 quarts/#, less if i need to free up mash tun volume for more grain. I collect runnings until I get my preboil volume unless the gravity of my runnings gets below 1.015 or pH gets below 5.6(never has). My preboil volume is my keg volume(5 gal)+ fermentation loss(1/2 gal)+ trub loss(1/2 gal)+ boil loss(1.2 gallon/hour). So, for my system, for a dipa w/ a 90 minute boil, I'd collect 8 gal. Since I fly sparged with 2 gallons more than you, I will have extracted more sugar.
On your Beersmith, under Mash Details, it tells you mash tun volume needed.