Thank you all very much for the replies. Second, please bear with me as I am quite new to AG. My first 3 gallon AG batch of Pale Ale was very close. The grain bill was as follow:
5lbs of Briess Pale Malt 2 Row
(the FGDB is 80.5% of 37 points or 29.7ppg)
.63lbs of Caramel/Crystal 60L
(the FGDB is 74% of 34points or 25.16ppg)
So, I calculated (29.78x5lbs)/3gal = 49.63ppg and (25.16x.63lbs)/3gal = 5.28ppg. Therefore, the OG should be around 1.055 based on the calculation. My actual OG reading was 1.056. My preboil gravity based on 75% efficiency should be 1.042 (1.056x75%) which was exactly the same with the actual reading.
The problem arose when I brewed an ESB yesterday (my 2nd AG batch). I wanted to brew a 3.50gal batch. With my equipment setup, beersmith suggested a preboil volume of 4.35 gal. According to the software, I should get a wort with OG of 1.059 and preboil gravity of 1.048 based on the following grain bill.
5.33lbs of Maris Otter
(FGDB is 82.5% of 38 or 31.35ppg)
1.06lbs of Biscuit Malt
(FGDB is 79% of 36 or 28.44ppg)
1.06lbs of Crystal 70L
(FGDB is 74% of 34 or 26.66ppg)
so if we were to use the above formula, I should get an OG of about 1.063.97 or 1.064 and preboil gravity of 1.048 (assuming 75%).
I only collected 4gal preboil using the suggested sparge water amount instead of the suggested 4.35gal. As a result, I only got a 2.5gal wort instead of 3.50gal. I lost 1gal to the 70mins boil and .50gal to cooling and trub.
My actual preboil gravity was 1.051 and OG was 1.070. Am I on the right track of formulating a recipe? Thanks again and sorry for the long post.