Noob question; ag to extract conversion

Sorry if this is obvious but I couldn't find it...i am looking at a mash/LT cooler conversion of the 5 gallon type and am interested in all grain brewing but I realize that to get good utilization I will need to maintain a good water to grain ratio. This presents a problem when you want to brew over 1.075 gravity brews. So I am asking how hard is it to figure out an extract replacement for an all-grain recipe trying to minimize the amount of extract used to maintain a good water to grain ratio in the mash vessel (5 gal total water+grain)?
Any good rules of thumb out there?

I don't usually brew many beers above 1.060, but generally speaking I'd start using extract once my mash tun can no longer accommodate the grist at 1 qt/lb. BeerSmith makes it pretty easy to figure out.