Adjusting for hop alpha acid rating ?

Planning on brewing up Jamil's California Common recipe from BCS book. The hop schedule is:

0.9 oz Northern Brewer (6.5% AA) for 60mins 23.5 IBU
1.5oz Northern Brewer (6.5% AA) for 15 mins 10.5 IBU
1.5oz Northern Brewer (6.5% AA) for 1 min 6.5 IBU

Total IBU is 41

But the NBs I got have an Alpha rating of 10.5%. Should I adjust my schedule? IF so, How should I adjust my amounts to get down to 41 ? Should I leave the third hop addition (1 min) alone and play with the other two additions? Or, just adjust the 60 min addition?

Thanks!

You only want to adjust the bittering additions. So, the first addition will change quite a bit but you don't have to adjust the later additions.

My Northern Brewers are 10.5%, too (did you also get yours from Hopsdirect?) and I used .75 ounce at 60 minutes.

That recipe is based on a full boil, of course. I assume you are doing a full boil, or an all-grain batch?

Disclaimer: This response will most likely be confusing because its confusing me!

I get wildly different numbers using Beersmith by calculating using Tinseth formula. From Beersmith's calculations you should need the following. Please note, for all the calculations I took into account that your AA rating is probably closer to 9.32% as the hops are probably from last years harvest and stored in vacuum sealed packages.

23.5IBU = 0.67oz of 10.5%AA for 60minutes
10.6IBU = 0.66oz of 10.5%AA for 15minutes
6.5 IBU = 4.7oz of 10.5%AA for 1 minute

I wouldn't change the hop bill to this as it's too drastic of a change from your recipe. If it were me I would leave the 1 minute hop addition alone at 1.5oz as you won't be getting that much IBU from the addtion to begin with. For the 15 minute addtion I would reduce it to 1.0oz which using the Rager formula on Beersmith would bring you to 10.1IBU for that addition and very close to what you need. Finally, for the 60 minute addition I would use the 0.67oz calculated using the Tinseth formula.

It seems that for the recipe the 15minute and 1 minute additions were calculated using the Rager formula while the 60 minute addition seems to be in between what the Rager and Tinseth formulas calculate. Therefore, the numbers I gave you should be rather close.

Edit: I would go with what Yooper said about the 15 minute addition and ignore what I said earlier. Also I assumed you were doing a 7 gallon boil to become a 5.5 gallon batch.

Thanks for the quick response. I am doing an AG full boil and I got them a couple weeks ago from Rebel Brewer.

I also noticed the wild calculations Beer Smith came up with. I'm just going to change the first addition to 0.75.

I think that 0.75oz should be good and a lot more simple than the confusing crap I told you.