I recently did a English Coffee Porter & did 2 oz. of French Roast grinds in a doubled up muslin bag. Worked pretty well and was able to get a good amount of coffee in the end product. Was nice and smooth. I would say, as in making coffee, the darker the roast the less amount of coffee needed - so it really depends on how dark your roast is.
2 oz / 5 gallons = 1.2 oz / 3 gallons