Ring Spinning
Below about a hundred fibres in the cross-section, no setting saves the yarn.
— x
Total draft less the break draft
Effective fibre length is not the same as staple length or upper-half mean; it is the working length the setting must clear and is normally taken from the length distribution at a stated percentile, so a value taken from an HVI staple figure will be short and will make the settings look wider than they are. The limit irregularity assumes fibres of uniform linear density arriving at random; a real fibre population has a spread of fineness, which raises the true limit somewhat above the value here. The expected CVm uses an index typical of a well-run frame and is a target rather than a prediction - it takes no account of the roving quality entering, and irregularity in the roving passes through drafting largely unchanged. Total draft is mechanical and excludes any fibre loss in the zone.
Ring-Frame Drafting Zone Settings & Irregularity Floor — free, with the formula and a worked example, at Textile School.