I would really recommend getting a digital scale. I got a pretty cheap used postal one for like $5. You can look around online and usually find them pretty cheap. Especially if they are used.
Not only is it good to make sure your snake is a good weight. It's also fun to measure them as they grow.
My first snake was a very fast grower. At only one year old she was already almost 3ft long and over 100 grams (I can't remember the exact weight). Being that size at just a year old, I thought that she was going to be a huge snake. But no. She grew to 4ft by age two and was up over 200 grams. At 3 years old she was 4 1/2ft or so and over 300 grams. I've backed her feedings down to 1 adult mouse every other week because she was getting chubby. She is now 4 years old and hasn't grown much.
Compared to my other snakes who are on the same feeding, she was pretty strange for all that growing she did in her very first year. My other snakes have stayed pretty small until they hit 2.