After many years of riding in America with metric speedos I have a formula to make it easy.
If its 10 to 15 MPH corner do it at plus 40 KM's. So 15 mph is 55 km/h.
If its a 20 to 30 MPH corner add 70. So 30 is 100 km/h.
All others just add 100 km/h.
I hope this is helpful.