Actually that's a great question- the very large (old "breeder") rats aren't that healthy to feed your snakes (they're higher in fat) & we agree with feeding small rats more frequently for best results.
Here's a chart that may help -& for a mature snake, I'd also add that weekly may be too much as well- so don't be afraid to make individual adjustments- when you describe your BP as "round as a can of coke" that suggests to me that he may be overweight, which is no better for a snake than it is for any of us.
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