1.) Theoretical Extract in Points per Gallon (TE)
2.) Gallons of Finished Beer (GF)
3.) Typical Extract Efficiency (EE)
4.) Pounds of Grain Mashed (PG)
I tend to use Maris Otter or a slightly lighter British 2-row as my base. It is probably safe to assume that the theoretical yield of this malt is 36 points per gallon. I also know that I do 5 gallon batches, my apparent attenuation floats around 76%, and my typical extract efficiency is 67%.
First thing you're going to do is essentially just figure out your GUs for the batch. That's easy enough.
[(TE*PG)(EE)]/GF or [(36*10)(.67]/5 = 48.24.
We can just round to 48.
This tells you that if you mashed 10 pounds of grain, you could expect a wort of about 1.048 in five gallons.
Now, if we divide pounds of grain by 48, we get a number that can help us figure quickly determine approximate OGs for batches with more or less malt. 10/48 = 0.208333..., but we can call it 0.208, or even 0.21 if you'd like.
If I divide the pounds of malt that I intend to mash, I can get a good idea of what sort of OG my wort will have. 10/.208 = ~48. 8/.208 = ~38.5. 16/.208 = ~77. That is, mashing 8 pounds of malt will yiled an approximate OG of 1.039 in 5 gallons. Mashing 16 will yield an approximate OG of 1.077 in five gallons.
You could also use the calculation to see determine approximately how many pounds of base grain you'll need for an OG of, say, 1.094. Just multiply your desired OG in GUs by your number. Again, in my case this is is 0.208. 0.208*94= ~19.6. So you'll need about 19.6 pounds to get an OG of 1.094 in five gallons.Of course, this only works well if things in your system stay relatively the same. But there's nothing keeping you from having numbers for Pils malt, or Munich, etc. if you use those grains frequently. Just a little trick to help give you an idea of what you're OG will be like for a certain amount of grain without doing all of the other calculations.