Batch Sparging & Grain Adjustment

I have a quick question. I was on www.tastybrew.com and they have multiple calculators. One of them is a sparge calculator. The "problem" is that it tells me to adjust my grain bill by around 1-2 lbs, and when I want to do a double batch it's more like 3-5lbs of adjustment. Now supposedly this calculator is to make up for the lost efficiency in batch vs fly. I guess the confusion and annoyance comes with the fact that my recipe is already adjusted for a brewhouse efficiency which my set up is already at. All I want to know is how much sparge water should I use. On my first two batches I used 1.3 qt to grain ratio. The first one was a success, the second one failed when I did a double recipe. Failed in the sense that my batch only yielded 9 gallons instead of 11 gallons.

The question is: Do I go ahead and adjust my grain bill to conform to tastybrew's calculator (which is based on Ken Schwartz's calculations) and perhaps overshoot my OG? Or does some one else have a system that works well with a ratio of 1.25 qt or 1.3 qt per lbs of grain of determining how much water to use for a batch sparge?

Sparge water is really simple. Sparge water = (Preboil water - strike water).
Divide that answer by 2 for 2 equal batch sparges which is best IMHO. Your grain soaks up some of this water per pound of grain and so you can add that to your sparge volume and you should be real close to you preboil volume when you have finished sparging (See absorption formula below). Remember that you want to not sparge with too much water as you would have to boil it off or settle for a weaker beer.

A Simple formula to determine water absorption: Total LBS Grain X .20 (Grain-Absorption-Rate) = Water Absorbed. Use 0.20 as your multiplier, which states that each LBS of grain in a mash will absorb 0.2 gallons of water. This is an approximate amount. If you have more than 25 % of adjuncts that will not absorb water such as Roasted Malts, Then you will have less absorption in the mash.

I use Beersmith and so it tells me what I need to know.
WBC

sounds like tastybrew's calculator isn't very good. I've seen people fly sparge bitching about 65% efficiency, and batch spargers gloating about 80%+

You are right in thinking that you need to consider YOUR brewhouse efficiency and not their guesstimates.