80's sci fi short story/film

I know this is a bit of a long shot but I have been trying to track down this movie for many years. It was an old movie my brother and I used to watch on vhs as kids. The story was two lads try to produce the perfect girl with a potion they acquired, the keep pouring this potion and different girls appear, some fat and then one has just half a body on a dining table. The only quote I remeber was when they poured it for the final time one said "Not too much, not too little"

Not a lot to go on I know but thought I'd give it a shot.

Thanks