I have been estimating taxes/fees as 1% of fare per sector as a primitive rule of thumb when constructing *ONEWC* itineraries. When BA priced them for me I was amazed at how accurate my guesstimate was - biggest difference $62.

When I thought about it, my rule seemed so illogical that I tried using an average tax/fee cost per leg instead. But the results were not nearly as close to the actual quotes.