Calculated fields in Laravel and general PHP

In Laravel / PHP its easy to create accessors and mutators to transform or create dynamic fields from a database.

Although its too easy to duplicate this same logic throughout your app, when you are not accessing the same data through an ORM model (e.g. direct queries).

$order->totalPlusTax; // (total * 10%)

The logic to calculate this field should be written once, testable and tied into an ORM.

What are best practices or design patterns around this logic?