I have so far brewed 3 batches of beer. The first was an Amber Ale with an
expected OG of 1.042 and the actual OG was 1.038 (not too bad). My second
batch was a stout with an expected OG of 1.066 and the actual OG was 1.061.
My third batch was a Guinness Clone with an expected OG of 1.043 and an
actual OG of 1.032.

This last one really disappointed me as I was hoping that my brewing skills
have been improving with experience. I was using SUDSW to calculate the
expected OG, is there a way I can work it out by hand and compare?

During the brewing process, what will affect the OG the most? Could the
actual be lower than expected because I'm only doing a 3gal partial boil?

-Nick