Which "rule of thumb" do you use for prey size?

[MikeV]

When i get my snake I am going with 15% of body weight

Also, in the poll, 15% of people say they use the 15% body weight method.

[Crazy4Herps]

For now I go with prey around the size of the snake's widest point (starting a bit smaller than that and varying between individuals based on metabolism and feeding response), but as soon as I get around to getting a scale I'd like to start using the 10-15% rule.

[Stewart_Reptiles]

Prey item slightly smaller to equal the BP's girth size for growing snakes and nothing larger than a 4 to 6 weeks old rats for adults. (male BP get the younger and smaller ones and females get the older and bigger ones)