Calculations with data from another form

Calculations with data from another form

Say I have a Form-1 with fields:
  1. Transaction-Date
  2. Product-Number
  3. Product-Price
  4. Transaction-Currency
and Form-2 with field:
  1. Date
  2. EUR
  3. GBP
where values in EUR column are x-change rate from EUR to USD.

How can I add a formula field to Form-1 where Price-USD = Product-Price x x-change rate for the Transaction-Currency at Transaction-Date?

How is this done to get the value in Form-1?
How is this done in the HTML view?