tried to search but couldn't find a discussion on this before...

any easy way to find the most authoritative answer to the total flight distances that's used in deciding 22k vs 26k? because, I tried the following itinerary, in all mileage calculators I found online, it's like ~15-20 miles below 22k...but my agent said it's actually 22,012.

HKG-NRT-LAX-SEA-PHX-TUS-LAX-SYD-HKG

I'm ok to change it a bit to fit under 22k but just because my agent does not always have the right answers to such more tricky FFP/airline questions...just want to make sure they're indeed correct. I'm ok if the official / authoritative distances used are somewhat different, but just want to see if there's a way for me to double check myself (besides calling up the airlines which might not be straightforward in finding the right ones who can answer the question etc).

thanks a lot !!