I don't know about feeding based on the size of the head, I go by girth. I feed my boas a meal that is almost as girthy as they are once a week. When they reach 4ft I will be switching to feeding them once every 2 weeks. This is the feeding advice I got from the breeder of my first boa and it's been working for me so far.

What snakes are you interested in that need to be fed pinkies for life? I don't know any that stay that small, even my little adult male kenyan sand boa takes hopper mice.

I also wouldn't recommend gerbils. Those are an expensive diet, you will probably be better off with rats. I know BPs tend to stick with one type of food and I believe boas are much more flexiable but rats are much cheaper than gerbils.