I just had an article published at The Hardball Times where I suggested a ERA estimator with GB% included could be the best yet. Here is some additional work I did since then.
Little more info: GB%: % of pitchers with ERA-FIP >1 SD under the mean (2012-2014, min 180 IP) <40%: 30% 40% to 55%: 11% >55%: 25%
— Jeff Zimmerman (@jeffwzimmerman) December 4, 2015
Image in first tweet for ease of viewing.