Is there a formula for adjusting HOP quantities when doing a late addition of extract
For example: If I start my boil adding only half of my LME, and dont add the other half until I have 15 minutes left on my boil, how do I make an adjustment for the increased hop utilization.
See Norm Pyle's Hops FAQ
However, you will probably find it a lot easier to get some brewing software Beersmith, Beer Tools Pro, Promash etc, or look for an online calculator (google IBU calculator).