It really depends on where you use it.
In the European Union, using a smartphone/tablet for the calculation of the fare is not allowed, and the hurdles to get an app approved for this functionality are purpously unsurmountable. Noone has dared trying it, but some well-known peer-to-peer ride-sharing platform ignores the law and has such functionality built into its driver app.
If you are a Limousine company, chances are very good that you can use Taximeter-GPS without infringing the law.
In the United States and Canada, using an app like ours has been made possible by the Authorities in many places. In Washington DC, the Regulator has gone that far as to decide that all taxis must be equiped with a digital meter app.
We suggest that you check directly with your local Authorities about your specific situation.