Betfair back / lay calculator

I need a back / lay calculator designing that will recalculate back / lay odds once a Rule 4 has taken place in horse racing. The user should be able to enter the existing odds they have taken from the bookies / Betfair. They should be able to select the rule 4 amount (which will give them the updated odds) and the revised Betfair lay odds and then calculator will work out the stake they need to place to get them in to the correct position. I would like this to be made in java script format. I will only be selecting somebody who has done work with Betfair applications before.