Backsalting is turning the freebase that has already been pulled back into a salt. Making it water soluble and sticking to your acid water. After this The solvent no longer holds any spice., and can be reused many times over. Its a defat basically. Fresh fluid is always used to pull for your dish.
It takes a system really.
If you were to go by a ratio, I think it's based more on how many ml of acid water than grams of bark. I don't remember the exact ratio. I've always used 50g to 75g for a 1 liter run. I'm rarely exact.
Oh yeah and when acid cooking, Allow that pot of stew that's been boiling cool down in the fridge overnight. Sediment will clump and you can pour good juice away. And recook your bark fiber. I would do it a few times. Then reduce all your solution.

So after the backsalt the solvent from the pulls can be reused? Isnt full of plant fats? Or could i do a sodium carb wash on it to clean it then reuse?

Tyou could but its not really necessary unless you've got your fluid really dirty