For me, I do partial mashes, partial boils with late extract additions. Since I begin my boil with just the contents of the mash and add the extract with 20 mins remaining on the boil, my hop utilization varies greatly. In my brew spreadsheet I made, I spilt each hop addition up for the amount of time it is at each SG of boil. For example, 60 mins bittering addition, I use 40 mins of the hop at the SG of the partial mash, and then for the remaining 20 mins use the SG of the partial mash and extract added. I use the following
formula from Palmer's book in my spreadsheet.
Utilization = f(G) x f(T)
where:
f(G) = 1.65 x 0.000125^(Gb - 1)
f(T) = [1 - e^(-0.04 x T)] / 4.15
As cweston said the BU:GU ratio is the key reading. After my IBUs are calculated on my spreadsheet I make sure that ratio is where I want it for the brew.
If you're curious how much of a difference it makes, spliting the hop times up and just using something like the Recipator that figures all fermentables in the kettle the entire time. For lower gravity beers not too much of difference, generally 5 or less IBUs. But for the SNCA clone-ish beer (high OG and high IBUs) I just made, it made a big difference something like 12 or 14 IBUs.
Probably more info than you wanted, but there it is anyway.
