Opening opposite positions (Choosing the right size)

 

Many mite not be aware that as the price rises, the value of a 'lot' increases.

For eg.

If a buy 1 lot  USDJPY is opened at 100.00, and now one wants to open a hedge sell at 105.00 (so as the equity going forward remains constant)

What sell size should be opened? Any mathematical formula for this? Thanks.

Reason: