Trouble understanding a recipe

Hey guys,
So I've baught the other day "home brewing beyond the basics",a book by Mike karnowski, by the way a great book in my opinion,but I'm having trouble trying to figure out the recipes in it especially the hop bill. In the book the autor mentions that instead of using ounces he prefers using alpha acid precentage to be more accurate. For example in one recipe he calls for 8alpha acids of target hops (35 ibu) without mentioning any ounces/grams. Now target hops have between 9-12% alpha acid, how can I calculate the hop quantity. I'm new to all grain and hoping this isn't a silly question, but I truly didn't understand how to calculate that using this system.
the funny thing that makes it more confusing, is that in some other recipes the hop quantity is mentioned in grams + alpha acid.
Hope someone could help, lots of thanks from advance,
Osher

Because of the variations in AA% between hops, going by IBUs is the only way to maintain any semblance of consistency for the end result of a recipe, so that's why you'll often see the hops given in IBUs. By the same token, grains will often be given in %, rather than a weight. The way to set up a recipe for your brewing system would be to use a recipe program, such as Beersmith or go online and use the free Brewer's Friend recipe calculator.

Here you go.
http://howtobrew.com/book/section-1/hops/hop-bittering-calculations

Thanks guys for your replays, but still on both examples an amount of hops is given. Im only trying to extract from that recipe the amount of hops in oz, but the only parameter that was given to me is the hop alpha acids in precentage and nothing else. The hop bill as follows;
8alpha acids of target hops (35 ibu) , how should I approach this to get the amount I need? Sorry if there was a misunderstanding regarding your answers.
Thanks again totally appreciate your feedback, for Brewers friends and the explanation above, hope you guys can help again cheers

I just read the intro to the book on Amazon, he has his recipes in AAU's (alpha acid units), which the link above posted by raysmith explains. Apparently it is also explained on page 77 of your book.

You need 8 AAU of target hops - so divide 8 by the AA% of your particular hops to get the weight in oz. For example if your hops are 8% aa then it is 1 oz, if they are 10% aa then it is 8/10 or 0.8 oz.

You made day brother, thanks a lot for digging in, and thanks a lot for all the helpers and reply ears have a beautiful day