CAC40 vs Eurostoxx Spread:
CAC40 (3540) x Multiplier (10) = 35400
Eurostoxx (2635) x Multiplier (10) = 26350
In this case, the perfect spread will be: 10 x CAC40 vs 13 x Eurostoxx (High Margin Requirements, perhaps). You can also use 1 x CAC40 vs 1 x Eurostoxx
Similar using CAC40 vs FTSE Spread, but not forget to cover de €/& change
Note1: The ratio of spreads is not a constant, it´s depend on the index prices and it changes every day
Note2: The index prices used in the example are approximate