Well I'm afraid you have me there, I was buying the ingredients for an IPA on the fly at my LHBS and wanted a full ounce of Chinook but they didn't have the plugs I wanted to use so I picked up a full ounce of Columbus instead. I haven't calculated the final recipe/amounts yet but if the only concern with bittering hops is the IBU contribution there shouldn't be a problem. My Chinook is 14% and my Columbus is 14.4%. I'm not even concerned about the total IBUs at this point as long as it's in the ball bark of 60-65.
The only sure thing of my recipe is the malt bill: Seven lbs. of pale LME and 2 lbs. of Munich LME in a three gallon boil with a late addition of half the LME.
So I either use the .5 oz of Chinook and .5 oz of Columbus if whatever flavor contribution they have is compatible (which I believe they would be) or I use all Columbus for the sake of consistency.
Cascade will make up the flavoring additions so the final recipe should be something like this:
7 lbs. pale LME
2 lbs. Munich LME (half of LME added at flameout)
1 lb. Crystal 60L
1 oz. Columbus (14.4%) 60 minutes
OR
.5 oz. Chinook (14%) and .5 Columbus (14.4%) 60 minutes
2 oz. Cascade (7.9%) 15 minutes
or
1 oz. Cascade (7.9%) 30 minutes (haven't decided yet)
Fifteen minute or Five minute Cascade addition??? (decision based on previous addition)
Dry Hop addition???
As you can see I still have a little work to do putting the recipe together but at least I have all the ingredients I need!