Partial mash is the way to go on big beers like this. I used a calculator, started with a couple of jugs of MO LME, then mashed the remainder to hit my target. LME, luckily, comes in known volume jugs, so I just figured that when calculating sparge volume. That being said, if you overdo it, and have a high volume, just boil longer. It will probably yield a higher efficiency.