Ok, let's do some math...

The original recipe called for 28 IBUs and a FG of 1.011. So, 28/11 = 2.5 IBU per point. In actuality was 28 IBU and a FG of 1.017. This is 1.6 IBU per point.

Now I want to get it to the 2.5 per point, which means I need to get to 42.5 IBU, adding 14.5 IBU.

Here's where I'm struggling. I have about 4.75 Gallons of this in the keg. Playing with an online calculator, I said .25 gallon boil for 15 mins, with a OG of 1.000 (water), using half ounce 10% AA hops. This gave an IBU number of 295, when diluted (divided by 20) is 14.75 IBU, which is pretty close to what I want.

Am I even close on this math and process?