Partial Close All - why does the system always close the oldest positions first?

[DaxVader]

I've asked the Help Desk and searched the forums, but I can't seem to find an answer.  If you have multiple positions open on the platform and choose "Close All" from the positions window, you can either close the entire position or partially close the position by adjusting the closing size.  However, the projected P&L is never accurate, because the system closes the oldest positions first, meaning you are left with a different average opening price and a different P&L than what was indicated.  It also means that you may be left with an open position that is in a loss, despite the aggregate position previously being in profit.  

For example, you have multiple positions in profit for £500 and you want to close 50% of the position.  You choose "Close All" and select 50% of your total size.  The system indicates you will be left with £250 P&L.  However, once you click close all the second time, the end result is completely different, because the system does not close 50% of all the open positions, it closes the positions you opened first until it has reached 50% of your total size.  Anyone know why and is there any way to change this? 

If I want to partially close a position and keep the same average opening price, there seems to be no way to do so other than to figure out the amount I need to close from each position manually and partially close them individually.  

Thanks!

DV