First off, if you're doing a partial boil, adding all the extracts at the start (before or at the boil) will lead to less hop utilization. (short answer - higher gravity wort with less volume can't absorb as much alpha acids)
That said, when I do extracts/partial boils, I add my extracts after steeping the specialty malts before the boil. I find I get to boil faster this way and avoid scorching. And, I'm not too concerned with the precise utilization when brewing this way.
Atomic Dog Brewery
On Deck: Centennial IPA, Dry Irish Stout
P: Cascade Pale Ale
T: Carbed Crystal Light, Root Beer, Vanilla Cream Soda, Tripel IPA, Apfelwein
e^(pi*i) + 1 = 0