Methemoglobin saturation is expressed as the percentage of hemoglobin in the methemoglobin state; That is MetHb as a proportion of Hb.
Less than 10% metHb - No symptoms
10-20% metHb - Skin discoloration only (most notably on mucus membranes)
20-30% metHb - Anxiety, headache, dyspnea on exertion
30-50% metHb - Fatigue, confusion, dizziness, tachypnea, palpitations
50-70% metHb - Coma, seizures, arrhythmias, acidosis
Greater than 70% metHb - Death