Slightly under feeding is better than slightly over feeding.

Here is a time proven formula that will take every bit of the guess work out of the question.

Get a scale
Weigh your snake in the unit of your choice (I prefer grams)
Right that number down
Determine what 10-15% of that number is
Find a rodent that weighs the same as that number or as close as possible
Feed one of these to your snake every 5-7 days

It's fool proof