It's going to vary a little from bike to bike. Not everyone's reads the same with the same sprocket combo.
The best way to calculate how far off it is is to use a GPS. Plenty of free speedo apps for phones.
If you want it to read correctly or closer you'll need a spedo drd or other brand of Speedo calibrator
Just pop a wheelie past the speed cameras and theres no need to look at the speedo!!
I've got same sprockets as you on 520 chain and seems to be 4-5 mph out. I just got my mate to ride next to me to check. It then turned into a race but that's a whole other story lol