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.
And they can give nasty bites!
0.1 Pastel Lesser Platinum (BP)
0.1 Dumerils Boa
0.1 Indian Sand Boa (Sunset)
0.1 Kenyan Sand Boa (Anery)
0.1 Kenyan Sand Boa (Rufescen)
0.1 Kenyan Sand Boa (Paradox Albino)
1.0 Kenyan Sand Boa (Paradox Snow)
And a lot of Tarantulas