All of the above. I have done something like that to "restore" carbonation to bottles that were a bit undercarbonated. Roughly, you need to ferment about 1 SG point to generate 0.5 vols of CO2. This means that starting at 1.005 and finishing at 1.000 will give you around 2.5 vols of CO2 which is about where most people aim for in a sealed bottle.
A SG of 1.005 is a concentration of about 13g of sugar per litre, so, for a 12 fl oz bottle (or 330 ml if you think in metric) you need something like a third of this which is about a teaspoon (4.5 grams)... good guess on your part!
It is easier to add sugar to bottles in liquid form, so a sugar syrup of 50/50 sugar and water makes life easy, you just add it with a (graduated) eye dropper. Five grams of sugar occupies 6ml, so 12 ml (0.4 fl oz) of 50/50 sugar syrup will add about a teaspoon of sugar. I am assuming that like most of us you are just after something with the right amount of fizz rather than some precise amount, so the above approach should give you a result that is "near enough". Even if you overcater, at normal room temperature even 4 or 5 volumes of CO2 won't generate more than 80 psi which is well within the normal limits of bottles (but might create "a volcano" when the bottle is opened!)
You can always try a bottle and measure what SG change you get when the sugar syrup is added, then adjust the amount to get 1.005 if needed, or even batch up the amount of cider that you want to carbonate and add sugar to that before bottling.
I guess the proviso with this approach is that we assume that because it is usually somewhat aged when it is bottled, cider has minimal CO2 in solution (unlike beer, which can have something approaching 1 atmosphere because it is bottled "fresh"). However this seems to work for me, in any case once it is poured cider gives up its CO2 quite quickly.
None of this is "rocket surgery" once you understand what you are trying to do. You can do some more precise arithmetic but it probably isn't necessary... Have fun!
