The amount required depends on the volume being carbonated. 1L of CO2 requires about 3.5 grams of Sodium bicarbonate which is about a teaspoon. I use Potassium bicarbonate (usually used in home brewing for controlling pH) instead because 1) it works just as well and 2) it boosts the amount of Potassium rather than Sodium in my diet. As for headspace, you only leave enough that the reaction doesnt cause the contents to splatter everywhere before you can recap the bottle. The contents should be sweet enough that it doesnt matter if theres a couple grams of a Sodium or Potassium salt mixed in which isnt hard to do if you use a reasonable amount of sweetener eg. table sugar, fruit juice, stevia etc.