Beersmith and those other links don't always have the correct SG potential for the specific grain you're using. I like to go right to the source...the maltster. Get the Fine Grind Dry Basis % and the moisture content % to calculate what 100% extraction would be for that certain grain. Also, for corn sugar, beersmith lists it as 1.046...the stuff at my lhbs is 1.040 when you put a pound of it in a gallon of water.
For example, Briess Pale Ale Malt
Extract FG, Dry Basis = FGDB = 80.0%
Moisture = MC = 4%
SG = 1 + (((FGDB/100)*((100-MC)/100)*46)/1000)
= 1 + ((.800*((100-4)/100)*46)/1000)
= 1.0353 for 100% extraction
In a spreadsheet, I calculate what 100% would be and compare that to the gravity sample after lautering. Also, when taking your volume measurement after lautering to use in your efficiency calculation, keep in mind that water expands as it is heated. For example, 9 gallons of water at 68F heated to 170F (mashout) will be ~9.107 gallons which will change your efficiency calculation by 1%.
Fermenting: JeanBeer, Barrel Fermented Dreg Lambic, Brett Trois Helles, Carrot Blossom Cedar Mead
Drinking: RIS, Coffee RIS, Jalapeno RIS, Heavy Hopper v2, Dusseldorf Altbier v5, Oktoberfest v5, Dopplebock v2, Cider, Rye Berliner Weisse, Sauerkraut Fermented Gose, Lambic, Brett Trois IPA, Brett Blonde, Kriek, Saison, Sour Blonde, RIS
Barrel aged: RIS, Rye Barleywine, Tripel, Wee-Heavy
Reason: Updated Briess link and math