Most brewing software is intended for all grain brewing, and as such can give some wonky results if large proportions of extract are used. Because of this it is sometimes a good idea to run through the calculations by hand, if for nothing else to check the software for possible errors. The calculations are very simple, and take about 30 seconds…
The most important thing is to get your Batch size right…. It doesn’t matter what the boil volume is, or how much water you top off with, just get the final volume correct or your results will be off no matter how they are calculated.

Potential Gravity Points:
Most all DME has a potential gravity of 1.044
LME varies from 1.033 to 1.037… try to find out what your supplier claims, or use 1.035 and adjust based on results.
For the calculations convert the potential gravity to “points” by sliding the decimal point over 3 places to the right and losing the “1”…. For example, 1.044 = 44 points. Pretty easy huh? (if you can’t do that in your head you may want to just go out and buy beer…)

If your recipe calls for 6-pounds of DME in a 5 gallon batch…
(Pounds/batch size) X potential gravity points
6 pounds/5 gallons = 1.2 (pounds per gallon) X 44 gravity points = 52.8
If that is all that is in there, move the decimal point 3 places to the left and throw a 1 in front… 52.8 = an O.G. of 1.053 (yeah I rounded it off)

If you use a combination of LME and DME like I used to do, just add the resulting points up before you convert to gravity.

It’s a little more involved if steeping grains are used, but not by much…
Most Crystal Malt has a Potential gravity of 1.033, so we are talking 33 points. (always a good idea to use what your supplier claims if you can get this information)

If your recipe has a pound and a half of Crystal in it:
(1.5/5) X 33 = 9.9
Now you have to take efficiency into account… I found that I got a 73% efficiency with steeping grains… (your mileage may vary, see what your measured gravity is with a few batches and back into the calculations to figure efficiency and average them)… convert the percentage to a decimal and multiply it to the points from the grain… (you didn’t need to do this for the extract, because it has already been “extracted”, and it is what it is)
9.9 X 0.73 = 7.23

(if you use a combination of steeping grains, do the same calcs for each type and add the points together)
If this was added to the boil pot with the above example of 6-pounds of DME, just add the points together:
52.8 (points from DME) + 7.23 (points from the crystal) = 60.03 (total points for recipe)
Convert this to gravity: 60.03 = an O.G. of 1.060

Done!
And that, is all there is to it.