I have a recipe that calls for 10ml of hop extract at 90 minutes. My local homebrew store does not sell hop extract. Since it is a bittering charge, what is the calculation used to convert 10ml of hop extract to alpha acids needed from pellet hops? IBUs are measured at 90 for the recipe. OG is 1.082. Recipe is here.
There are plenty of online IBU calculators that will help you out, such as this one.
The recipe calls for adding 4 ounces with 15 minutes left in the boil, and 3.75 at flameout. Assuming the flameout addition adds no bitterness, we still have to calculate the IBU contributions from the 15 minute addition. Let's assume these AA percentages:
Using the online calculator, and assuming pellet hops, this yields around 50 IBUs, meaning we have to get another 40 IBUs from a 90 minute addition.
You can use any high-alpha hop, as the 90 minute boil means that very little varietal flavour will remain in the beer. For example, another 1.1 oz of Simcoe gets you there.
Once you've purchased your hops, check the AA percentages and re-run the calculations. Different batches of hops can have very different AA percentages.