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You're right, since looking at it again, ERA+ is LgERA/ERA, expressed as a percentage. (Normalized for park, of course.) So:
AL ERA 2009 = 4.46
150 ERA+ = 4.46/2.97 = 1.5
50 ERA+ = 4.46/8.92 - 0.5
Lessee, just real quick here, to "cancel" a 150 ERA+ of 2.97, well, 2.97 is 149 points lower than average, so we need 4.46+1.49 = 5.90 ... bravo! 4.46/5.90 = .75.
Right you are. So to "cancel" 80 games' worth of 150 ERA+ starting pitching, you need 80 games' worth of 75 ERA+ starting pitching. Thanks for the correction.