I still dont really understand???

I still dont toally understand how to calculate mash efficiency when doing a partial mash. I just got done brewing and I am very happy, I hit the OG# that the calculator I was using said I would, I just dont know how to calculate the efficiency of the mash. I used 4 lbs of DME, as far as the grains they were
3 lbs of British 2-Row
.5lbs of Belgian Biscuit
.5lbs of Belgian Choc.
6 oz of Belgian Special B
and I added 4 oz of lactose

Boiled 3 gals, and batch was 5.5 gals total.

OG was 1.052 the online calc I used to make the recipe had this # as the OG so I figure I was around the 75% they say, I just dont understand how. Oh well at least I think I had decent effciency.

Have you read Palmer's explanation? http://www.howtobrew.com/section2/chapter12-3.html
Read that and the next few sections. ;-)

Me and math don't get along so I went out and got ProMash.
I just ran your recipe and gravity through it and I come up with about 66% efficiency. Not too bad.

Thanks Rich, I guess that isnt too bad considering it was my first good PM. The other I tried, the grains were not crushed well, and I later found out they were pretty old. My one problem yesterday was that my mash temp was only 147. I imagine next time it will be better with the temp 152-155 like it should be. I downloaded a trial version of promash but dont really see how to use it and there isnt any directions. I am going to have to buy something soon though to understand this mash efficiency stuff.