It doesn't matter if it's a full wort boil or a part wort boil, the hops utilisation for a 1.080 wort will be the same.

If doing a part wort boil, it's really simple to work out your additions. For example you're making 20ltrs of a 1.050 beer with 25IBUs, using 1 addition at 60 mins and the hops are 4% AA.

If it was a full wort boil you would use the utilisation figure for 1.050 which is 23.1%, so you would need 54gms of hops. But you are only going to boil 15ltrs and then dilute it to 20ltrs. To do this you will need 15ltrs of a 1.067 wort. So what you do is work out how much hops you need for 25IBUs in 20ltrs but use the utilisation for 1.067, or simply 1.070, which is 19.3%, and therefore you will need 49gms of hops.

You will end up with exactly the same amount of isomerised alpha acids by boiling 54 gms of 4% AA in 20ltrs of 1.050 wort for 60mins, as you do for boiling 49gms in 15ltrs of 1.067!

Again this is a simplified account, becuase as the wort boils and concentrates the utilisation is not a linear decline, but the basic principles are correct.

I know there are many programs that can do these calculations for you, but by learning the maths behind the programs, it will give a better understand what is happening and you are more capable of making adjustments on the fly.

Cheers

MAH