Whats the formula does the HDI use to measure life expectancy?

The formula tells you how to convert life expectancy into a life expectancy index. For example, Turkey had a life expectancy of 71.4 in 2005, so the life expectancy index is 71.4-25/(85-25) = 0.773. The index does not imply that countries cannot have life expectancies smaller than 25 or larger than 85 - if the LE is larger than 85, the index value will be higher than one. If LE is smaller than 25, the index value will be negative.