Synthetic Instrument Calculation Error

 

I've created a really simple synthetic instrument (SIEN20 - XAGUSD) which shows spread between Silver and its Sep futures contract. Works perfectly in strategy tester and I can easily do analysis and create my own trading logic using it, but when it comes to real live trading, it keeps having calculation error and I'm not sure why. Anyone can share their thoughts and perhaps share potential solutions so I can fix this? For example, right now the spread is at around 0.3 but in the graph it's showing 17.845...



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