I want to brew a 2.5 gallon batch of beer based on a West Coast IPA recipe for a standard 5 gallon batch. Is there a calculation for scaling back the amount of hops used for bittering, aroma and dry-hopping? I assume one doesn't add the same amount of hops as in a 5 gallon batch, right?!
You can simply half the hops for a half sized recipe. Although hop utilization is non linear, at these boil sizes the difference is negligible compared to other factors, such as alpha-acids lost due to aging.
For flavor and aroma hops, their contribution is also related to batch size and quantity of hops, so again, these can be simply scaled in proportion to the recipe size.