The spread ratio of exchange given is not always correct, because prices change all the time. Not just index future i wanted to trade, but this also applies to all futures out there as wellYou didn't indicate which futures you want to spread. But since you posted in the Index Futures forum, there is a good chance that the index futures spread you want to trade may offer margin relief if the exchange recognizes the spread. In this case, use the ratio specified by the exchange and forget about IV.