So, I used a recipe formulator (beer recipator) and calculated an OG of 1.066. When I brewed, I took a reading from the kettle, then after racking to the primary I had to top-off with 0.75 gallons of water.

Reading was at 1.072. So, is this formula correct for calculating real OG?

[4.25(1.072) + 0.75] / 5

because that equates to 1.0612, which is a bit lower than expected OG.

I used 7 lb of DME in a 6 gallon boil. I did leave some behind in the kettle, for the trub. Is this where my missing gravity is?