1, only when the specification is limited to one side: horse level z = zbench+1.5 = (USL-μ)/σ+1.5, where: zbench = (USL-μ)/σ.
2. Only when the specification is limited on one side: Sigma level z = zbench+1.5 = (μ-lsl)/σ+1.5, where: zbench = (μ-lsl)/σ.