Recipe Design: Hops

It seems that there isn't really a consistent AA number for hops. It could vary depending on the vendor or the time of year. Depending on your hop schedule these differences will potentially make a big difference in the final product. How do you work your recipe around this?

Do you adjust your hop totals to get the same IBUs or do you just keep the totals consistent?
How does this effect the later additions where the bitterness is less important than flavor or nose?

Just me personally but I've gotten to the point that my bittering hops are very interchangable..ie....I will just plug the most effecient (read high AA hop I can find) at the moment Northern Brewer in as my bittering hop...and then the flavor and aroma hops I follow the recipe or style guide as close as I can...the reason being that.
if you know what your total IBU's are supposed to be and you know what your flavor and aroma hops add...then you work backwards...ie...if you get a batch of Northern Brewer at 9 % AA then you set your recipe accordingly and set the right amount...but if your bittering hops are 14% of course you'll need less...and good software of course will tell you when your on target.

If that isnt clear pm me and I'll expand.

Cheers!

I just scale my hops additions according the the AA%. However I also buy in bulk and store them in vacuum packs in the freezer, so my AA% is pretty consistent from batch to batch over long periods.

This is why recipe calculation software is so useful. That way you can account for the differences in your bittering hops. Anything past 20 you probably don't have to worry about.