Your smoking habit is the best guide. Here's a rough map:

• Never smoked, or only occasionally? Start at 0 mg/ml or 3 mg/ml. You're vaping for the ritual, not the nicotine.
• Light smoker (under 10 a day): 10–15 mg/ml salt, or 6 mg/ml freebase.
• Moderate smoker (half a pack to a pack a day): 20–25 mg/ml salt.
• Heavy smoker (more than a pack a day): 30–50 mg/ml salt, but start at the lower end and adjust.

These are starting points, not rules. Your body will tell you if you overshot—usually a scratchy throat or a mild headache. That's a sign to step down, not to push through.