Where did the >50 EBP = OK for vented heuristic come from?

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Advice seems to be unanimous across the web that an EBP of 50 is an appropriate cutoff where <50 means only suitable for sealed while >50 is OK for ported/vented, or maybe at higher points even for horn application.

Where did this originally come from, or how was it derived? It doesn't seem obvious at all to me that this is a better way to determine whether you can go ported than just eyeballing Qts (lower is better) and BL (higher is better). Ideas?
 
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