Use Workflow or Formula Field for VAT Calculation

Use Workflow or Formula Field for VAT Calculation

Hi,

I need to calculate the VAT based on the rate selected in a dropdown, so far I have managed to calculate the Net cost based on the Unit Cost multiplied by the Quantity. The VAT dropdown has 3 options, 0, 0.05, 0.20 depending on which is selected the Net Cost will be multiplied by the VAT however I'm running in to syntax errors

My field names are...

Qty
Unit Cost
Net
VAT
Total VAT
Gross

Any help with this would be great as I'm really struggling to get this working