I'm assuming you mean you're starting AG (all grain) next week. You want to keep a light DME (dry malt extract) on hand. It will allow you to do gravity adjustments using the following formulas from Ray Daniels' book designing great beers. Here are the formulas and then I'll explain them:
Formula 1: Preboil GU x Preboil volume / desired postboil volume = the gravity of your wort after concentrating the sugars through boiling.
I need this formula to figure out what my gravity will be when I'm done boiling. I may be spot on and no gravity adjustment is needed. I may be way off and need to use DME in the boil to hit my desired gravity.
Before I break down the formula let me explain what a GU is. It stands for gravity unit. Its essentially the last two digits of your specific gravity. The GU of a 1.050 beer is 50. The GU of a 1.085 Dopplebock is 85. Mathematically its ((specific gravity x 1000)1000), but who likes math.
The first part of the equation comes from values you will measure after you finish mashing and run everything into your kettle, BUT BEFORE YOU START BOILING. So you're measuring your gravity of the total volume you collected in the kettle (don't take this reading from a mash tun sample, you're gravity will be higher at the beginning of run off and lower at the end of run off). You're also measuring the total volume you collected.
Let’s say you’re readings here were 1.048 (or 48 GU) and 7.25 gallons wort. 7.25 x 48=348.
Now divide 348 by your desired volume after you’re done boiling. For me this is 6 gallons (as an aside I leave 1/2 a gallon behind in the kettle, move 5.5 gallons to my carboy and after fermentation I leave 1/2 a gallon in the carboy and move 5 gallons of trub and debris free beer to a keg)
So now we have 348/6 = 58 or a specific gravity of 1.058 after the boil. Great… but I wanted to brew a 1.065 beer. So this is where we go to Formula #2. Note that you should have used formula #1 before you started boiling your wort.
Formula 2:
(Target GU x Target Volume)  (Preboil GU x Preboil Volume) / Extract per lbs.
In the first part of the equation I'm applying the volume I want and the specific gravity I want after the boil. So I want 6 gallons of wort after the boil and my recipe is a 1.065 beer. So my first number is 6x65 = 390.
So now we have:
390  (Preboil GU x Preboil Volume) / Extract per lbs.
The second part of this equation is identical to the first part of the first equation above. We already know that number, it is 348.
So now we have:
(390  348) / Extract per lbs.
Extract per lbs. for Briess light or pilsen DME is 45.
So now we have
(390  348) / 45 = 0.93
I need 0.93 lbs of DME added to the boil to hit my target gravity of 1.065.
