The preceding post is good advice.
As for your question, I don't think anyone really knows how the factory measures seat height, any more than we know how they measure horsepower or curb weight. Wishful thinking seems to be involved.
The height in inches you reported is almost certainly a conversion from 80.5 cm, measured by somebody using a tape, somewhere. It should be the distance from the lowest point of the seat to the ground, but how is the motorcycle to be loaded, if at all, before the measurement is taken? Should a carpenter's level be placed across the lowest point of the seat to provide a reference? (Yes!) Who knows?
Instead of trying to measure the actual seat height on the motorcycle, I think you would do better to directly compare the two seats themselves. Maybe you can sit on the same model with a standard seat if you are one of the lucky few who is near a dealer or fellow owner. Maybe you should take a photo of your seat from the side and compare it to photos of the factory seat. Maybe you can call the previous owner and ask him or her if it made "a big difference." If it made a big difference for them maybe it will make a big difference for you to go back.