If you only have an 8 gallon kettle it appears, without doing the math, that you would need to essentially halve (or 1/4) the recipe. With the 90 minute boil and amount of hops it appears the creator accounts for almost 2 gallons of boil off and trub loss (5.5 into primary, 5 gallons finished volume). Brewing software would make this an easy deal as you can create your equipment profile and then easily scale any recipe regardless of size.