For the few readers I didn't scare away with the title, here are a couple of good pieces of research done on pitchers. While the articles will fly over most peoples' heads, including mine at times, I extracted out the main points.
The ERA estimators that were tested (xFIP, FIP, SIERA and tERA) all did a better job of predicting future ERA than actual ERA; which was to be expected and is the normal assumption in the sabermetric community. But although they did better than ERA, simply subtracting walks from strikeouts did a better job of predicting ERAs for the second half than any of the advanced statistics.
My 2 cents:
With so much other noise surrounding a pitcher, it is best to just look at his K and BB rates when determining how he will perform over the course of a season.
To figure out his 10th percentile, we multiply by runs allowed [for the projection] index (70% in the case of this great pitcher we're illustrating), by 135%, ... The gap between the 90th percentile and the 10th percentile is 58% (135% - 77%) of whatever the pitcher's mean rate is.
My 2 cents:
Whenever projections begin to come, 80% of time the pitcher's (E)RA will be between 77% and 135% of the value. This should give people an idea of potential range for the pitcher's performance.