Forex Question: How to hedge spot forex with futures contracts?

Hello,
I'd appreciate your help here.
There is 1 BUY standard lot size in EURAUD (forex spot market).
How would you hedge it with futures contracts?
Simply SELL EURO + BUY AUSSIE, again 1 standard lot? I doubt so, as I tried it on demo accounts and after few hours I see it doesn't match.
What would be the right calculation?
Can it be 100% hedged or will there be always some differencies?
Thanks for your advice.

Source.