The deflection limits are usually based upon the amount of curvature in the beam - this goes back to when a lot of buildings had lath and plaster ceilings.
Too much curvature can distress the connections between the plaster and structure - or crack the plaster due ot the lengthening along the curve.
Since the limits for the curvature are simplified to be based upon the deflection as a percent of length (L/360 for example) this is based on simple span beam deflection vs. curvature.
A cantilever in deflection can be envisioned as a half length of a simple span beam. Thus, to equate the L/360 deflection limit you would use 2 x L(cant) to get an equivalent simple span beam length.
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