Additional Costs - Auto Calculate % of order

Looking to create a service product that is able to automatically calculate ‘x’ percentage to add or subtract on an Order. 

Eg. Sales order, requires a 10% increase for a tariff / charge. A ‘% Tariff’ code can be added into the additional costs section and it will generate a value based on the chosen percentage, of the order. 

Order showing as £100, ‘10% Tarriff’ would add an additional £10 onto the order.