Use which strike is closest to the index value and you look at it as a tool whenever you are analyzing strikes. It is just one of many tools to help you make strike selections. And yes a straddle is the Put and Call at the same strike price.
Quote from piccon:
What strike do you choose 1275 or 1270?
At what specific time do you take this measure?
Do you take the PUT and the Call and then add it up? (long Straddle definition)
Please answer.

It was an interesting (and hopefully profitable) experiment; much like your straddle play.