Odd... I've seen 540, 764, and 1080.. on an asian tractor.. but not 450.
If you needed 540, try the 450 setting, and figure the engine rpm to pto ratio, and then see if you can run in that higher range, as the pto rpm mark should be on your tack. If the range takes you into too high of an rpm setting, again, try the 7?? one.. and back down accordingly.. though this may drop you out of your power curve...
You sure about those settings?
Soundguy