Your tool is probably basing the water quantities on your target pre-boil volume. For a big RIS, you're going to leave a ton of sugars in the mash with that small of a sparge volume. You could add some more grain and do a partygyle. What I do for a big RIS (or any really big beer) is load up my HLT with a good amount of sparge water (I usually heat up 8-10 gallons regardless as is doesn't hurt to have too much), I'll sparge until I get a desired pre-boil volume (more than i normally would have and boil down) or until the runnings are at you're desired gravity (Typically 1.010). Since my system is set up to run from the MT directly to the BK, I don't typically test the gravity. I also know how much pre-boil wort I need to hit a certain gravity based on boil off (at least for my system). So for a 5 gallon batch of, lets say 1.100 OG wort, I woud shoot for a pre-boil volume of about 10 gallons (I'd have to check my notes at home to be sure), assuming no added sugars in the boil. I'd then boil down to 7 gallons, test gravity, and add my 60 minute hop addition, continuing as normal. By starting with more wort and boiling down, you're concentrating the sugars which will allow you to hit your high OG. Others may do things differently, however.