How much should I feed a dog with a very slow metabolism?

Before assuming a slow metabolism, rule out the more common explanations first: undercounted treats or extras, an overestimate of actual activity level, or a recent reduction in exercise that has not yet been reflected in the food amount.

If those are genuinely ruled out and your dog still gains weight on what should be a reasonable, correctly calculated amount, some individual dogs do have a lower than average metabolic rate, similar to how metabolic rate varies between individual people at a similar size and activity level.

Rather than continuing to use the standard population average multiplier, treat your dog's stable weight maintaining amount, found through careful trial and body condition tracking, as the real working figure specific to that individual.

This might mean using an activity multiplier closer to 1.3 to 1.4 rather than the standard 1.6, even for a dog that seems reasonably active, since the multiplier is ultimately just a starting estimate to be refined by actual results.

Once you find the amount that keeps your dog at a stable, healthy weight, that becomes the ongoing target, checked periodically since metabolism itself can shift further with age, health, or activity changes over time.

If maintaining a healthy weight on a genuinely low amount leaves your dog seeming hungry, consider a lower calorie density food that allows a larger comfortable portion for the same lower calorie total, and mention the situation to your vet to rule out any underlying cause.