oh yeah, sorry. Batch sparging
In that case:
1) Pick your batch size. For example, 5.25 gal. This is the amount you want to be sitting in your kettle at the end of the brew.
2) Figure out your boil-off rate. Depending on your setup, this could be anything from .5 gal/hour to 2 gal/hour. If you do 60min boils, add this value to your batch size to get your pre-boil volume. For example, if you get one gallon of boil off in an hour, your pre-boil volume is 6.25 gallons.
3) Typically, you want half of your wort to come from your first runnings and half from your batch sparge (assuming you are doing a single batch sparge). So, you should use half of your pre-boil volume (here, 3.125 gallons) as sparge water.
4) Your strike water will be equal to: half of your pre-boil volume + grain absorption + mash tun deadspace. Mash tun deadspace is the amount of liquid left in your mash tun after you drain it, and should be constant from batch to batch (i.e., a quart). Grain absorption will vary from batch to batch, depending on the size of your grist bill. Most people use .125 gal/pound as a good approximation, though other facts can to some degree impact this.
So, with any new system, you've got to figure out boil-off rate and mash tun deadspace. You should check your gravity pre-boil to know how your efficiency was, and then from there decide if you need to do a longer boil to get your water levels down properly. If your wort is too concentrated (i.e., your efficiency was better than you predicted), you can always add water. But, if you extend your boil after you've added all of your hops, you'll significantly change your recipe's hop profile.
