So if the spread is variable, how will I know what spread I will get?

The idea behind a variable spread is that it cannot be fully predicted when you open a trade.
 
For some instruments, the difference might be more noticeable than in others. However, in general, there is no guarantee regarding the spread when a trade is closed.
 
With that in mind, we always seek to provide the best rates available for our customers, and we work only with top tier liquidity providers for this very reason.