It looks like you will need to break it out into a bunch of rules, or set up a database to store the values.
I'd probably set up a database personally and use a total of three shipping rules:
Shipping rule 1: Total shipping returned from database query (return 0 if over $250)
Shipping rule 2: Domestic large order 7.5% if domestic session variable set and order total over $250
Shipping rule 3: International large order 10% if international session variable set and order total over $250
in your database table you would have shipping rates with columns like
ShippingID ShippingMaxWeight ShippingAmount ShippingZone
Shipping Zone would identify international or domestic and ShippingID is an auto-increment key field.