Margin has the same meaning in all platforms and with all brokers. It's simply the money you have to pay to the broker for buying 1 lot of the asset. So, with your 10000 Capital and at $200 SPY price, 0.1*(Capital+ProfitClosed) will initially buy you 0.1*10000/200 = 5 contracts.

I am sure that this math is also the same in all platforms and with all brokers.