How to calculate profit per trade?

Hi,

I'm struggling with what I thought would be simple. I'm trying to work out the % profit each trade has made when it is closed off.

However, using the Open API v1 I don't have a way of knowing what the balance was when the trade was opened. If I did, I could do the following calculation:

Balance when trade was opened = £10,000

Profit made on closing trade = £100

Profit % for that trade = 100 / 10000 * 100 = 1%

When I place a trade using the Open API, It does not provide me with the account balance at the time the trade is opened. It does, however give me the current balance when the trade is closed. But, if I don't know the balance the trade was opened then I don't see how I can work out the profit %.

What am I missing???

Thanks

Ro