What is the typical minimum underlying bodily-injury liability limit that an ISO-form personal umbrella insurer requires on the insured's Personal Auto Policy (PAP)?

Why this is the answer

Personal umbrella insurers publish a Schedule of Underlying Insurance specifying the minimum primary limits the insured must carry. For the PAP, the industry norm is $250,000 per person / $500,000 per accident bodily injury and $100,000 per accident property damage (or a $300,000 single limit equivalent). If the insured carries less, the umbrella treats the difference as a self-insured retention — the insured pays out of pocket up to the required underlying amount before the umbrella attaches. See ISO DL 98 01 Schedule of Underlying Insurance.