If you were to enter in the products_weight field: 2

and then set up the US to be:
1 x 2

and then set up Canda to be:
1.25 x 2

and then set up The Rest of the World to be:
1.75 x 2

where 1, 1.25 and 1.75 were "shipping units" ... would that work?

The Per Unit perweightunit is designed for this type of shipping cost where it can be cloned for each Zone and have a Unit Rate set ... so if the shipping rates were proportional, this should work ...

NOTE: in Shipping/Packaging the Maximum Weight needs to be set to 5000 or higher and the Tare amounts to 0:0 ...