Ya, I brewed another batch recently and I'm really enjoying it in the warm weather!
The 13.5% AA Magnum will work fine. You can adjust for the different AA% to get the same calculated IBUs by factoring in the ratio of the AA%:
(14.0/13.5) x 9g = 9.3g
So you would use 9.3 g of the 13.5% Magnum to get the same bitterness as the 14.0% Magnum i used. But practically speaking, this difference is so small and my scale only has a 1 g resolution, so you should be fine just measuring out 9 grams with the 13.5% Magnum. Yes, the Magnum go in first. The Magnum were First Wort Hops, which means I added them to the kettle when I started collecting my first runnings after the mash.
Your 10.3% Centennials will work just fine in the same amounts. Since they are added at the last 10 minutes and at the end of the boil they will not add much bitterness. These are for flavor and aroma so you should use the same amount (per 6 gallon batch).
Hope this helps!
Last edited by thunderworm; 05-27-2011 at 07:26 PM.