I usually give quantities of gelatine in terms of the powdered version, which comes in ½ oz/14 g packets, but if you prefer to use leaf gelatine you can substitute four leaves for ½ oz/14 g. This quantity is sufficient to set 1 pint of liquid.
A word of warning: if the weather is particularly warm, and if you are going to turn out what you have made, I advise you to increase the gelatine content. In hot weather a gelatine-set mousse or jelly very quickly ‘relaxes’ – or melts. Equally, left for any length of time in the fridge, gelatine toughens. It is as well to take a mousse out of the fridge and into room temperature for half an hour before serving it.