I've always thought brownie mix to be a good one to use.

I've found one box from sainbury's plus egg, water and a little oil (read the instructions...), and I use about an 1/8 to one box but be warned, they're f'ing strong and lasts abotu 4 hours of intense effects.

I'd say a good measure would be a 1/16 to one box if you're not a regular to the greenery...