What is the typical minimum underlying Personal Liability (Coverage E) limit that a personal umbrella insurer requires on the insured's Homeowners policy?

Why this is the answer

Although ISO HO 00 03 Coverage E defaults to $100,000 per occurrence, personal umbrella insurers typically require the insured to increase Coverage E to $300,000 (a routine endorsement available on most HO programs) before binding the umbrella. The Schedule of Underlying Insurance on ISO DL 98 01 lists this requirement, and failure to maintain it triggers the 'maintenance of underlying' penalty — the insured personally absorbs the gap. See ISO DL 98 01 Schedule of Underlying — Homeowners Section II.