You can re-use yeast from any batch. They just caution re-using yeast from high gravity beers as it has a higher chance of mutation. For some Belgians, Commercial brewers have to re-use from high gravity beers.
For re-use of slurry, I find Mr. Malty more of a guess and I don't agree with it's calculation of viability. When re-using yeast, I generally use about a quarter of the yeast cake for a beer of similar gravity. If the new beer is twice the gravity, I would use a half of the cake.
My reasoning for using a quarter of the cake: Generally, a finished beer will contain 6x the yeast that would have been the recommended pitching rate (this is true if you over or under pitched). I assume some will have died due to the weight of liquid on it in the fermenter. Thus probably a fifth would be correct, but I assume my collection and storage are not perfect, so I use a quarter. Never had a problem if stored in fridge and used within a month.