Gelatine is sold by its strength. This is known as "Bloom", starting at about 90 Bloom (the weakest) to about 300 Bloom (the strongest). The powder sold in supermarkets is about 120 Bloom.
Professional sweet makers etc use strengths between 220 - 250 bloom, while the 300 bloom (which I also sell) is generally used for special effects makeup, prosthetics and ballistics testing.
If you are thinking about the 240 bloom you will use between a half to a third less than supermarket powder. You will need to experiment with the quantity of gelatine to use but I would start with about half the amount that you normally use, and see if that is enough. There is no difference in taste.
Gelatine is also affected by what you are trying to gel, for example a liquid such as Lemon juice will require more gelatine to set, than if you were using just water.
You will also need to "bloom" the gelatine before you use it this means soaking the gelatine for 30 mins or more in 3 to 4 times its own weight of water, or other liquid, then heating it up over some form of double boiler, or in the microwave, until it forms a clear liquid. Never allow gelatine to boil, or use boiling water, as this affects the strength of the gelatine.