Protection isn't usually ever a factor. As far as having a metric to measure it's effect? My memory is that metrics tend to show that it doesn't matter how you construct a lineup in terms of the batting order and its productivity. Could be wrong, though. Actually, that was the perceived thought until 3 of the foremost sabermetric minds published The Book Batting order does matter. Having not read The Book, is it talking about lineup construction as it pertains to overall team production, or protection in the lineup leading to better performance for an individual hitter?