Function in Zoho Book

I am looking for a freelancer who can assist me with with a custom function to calculate selling price. I am not sure at this time if I will need future support or if this will be a one-time job.

Customs fields are already created
Price * Exchange = Rate (selling price)
Price and exchange value will be entered by users as the estimate is created result value need to be inserted into the rate field

There will always be only one item per estimate invoice.

Requirements for the project include
Zoho book
Deluge

If you have experience with Zoho Book and setting up the invoicing function, please submit your proposal.