Don't take my word for this because I am a newbie myself but I weighed my snake at 122 grams so I been trying to find food that weights about 12 grams I had heard 10 -15% of body weight and that may be light but it has been working for me. Since my snake is still small I have been feeding every 5 days.
After weighing a few mice you get pretty good at judging the size of them.