A media streaming company runs identical backend fleets in us-east-1 and eu-west-1, each fronted by a public Network Load Balancer. Users are currently routed to the geographically closest fleet. Ahead of a major regional launch, the operations team wants to gradually shift more traffic toward the eu-west-1 fleet to validate its capacity under production load, while still keeping routing fundamentally based on the physical location of both users and resources. They need a single Route 53 policy that lets them expand or shrink the effective service area of a region using a tunable value. Which Route 53 routing policy meets this requirement?