Chocolate poisoning is dose-dependent - a square of milk chocolate rarely harms a big dog, while the same amount of dark can seriously affect a small one.
An estimate to gauge urgency, not a diagnosis. Theobromine content varies and dogs differ in sensitivity - when in doubt, call your vet or a poison-control line with the packaging in hand.
Figures from the Merck Veterinary Manual: methylxanthines (theobromine plus caffeine) per gram of product; mild signs from about 20 mg/kg, heart effects at 40-50, seizures from 60.
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