How can I adjust a recipe for different grades/brands/types of gelatin?
Looked up 'gelatin strength' in Google and found this link. Very interesting.

There are two types of gelatin: A and B (I have no idea what the 'isoionic point' is).

Anyway, in the shop, I guess you'll find approximately the same 'strength' gelatin, independent of the format.

Normally, the gelatin sheets are 2g and the granular gelatin is packed in 6gr envelopes. 3*2=6.

6g of gelatin (1 envelope) will bind 500ml water for jelly (as per the instructions on the envelope). Substitute water for other liquid and you are set to go.

Edit: Personally, I don't think the precision is too important. This weekend I added too much water to a recipe (500ml) and the result was nearly perfect. That is, it didn't collapse but was hard to ration.

Also - Changed gr to g.