I came across this tool to calculate ZED fares including taxes. It is fairly accurate. Unfortunately, you need to be a member of one of the participating airlines to get access.

https://www.id90.com/interline_fare_calculator.shtml


===================Participating Airlines========================

Aeroflot
Air Caledonia
Air Malta
Air North
Air Tahiti
Alaska Airlines
American Airlines
Azul Linhas Aéreas
Cape Air
Colgan Air
Continental Airlines
Delta Air Lines
Express Jet
Frontier Airlines
Hawaiian Airlines
Horizon Air
Kenya Airways
Midwest Airlines
Mokulele Airlines
Papillon Airlines
Porter Airlines
SkyWest Airlines
Spirit Airlines
Sun Country Airlines
Trans States Airlines
Virgin America