I would like to have a form do the following automatically:

text_field_1 = (User Input) (e.g. 12.00)
text_field_2 = text_field_1/100*3.4+0.2 (e.g. 0.608)
text_field_3 = text_field_1 + text_field_2 (e.g. 12.608)

It is to calculate additional charges for accepting paypal payment. It would be great if it could automatically round up or down on the final value to give an accurate figure.

If this has already been done can anyone point in the right direction?