How to give each item in custom list it's own value range, with each value incrementing by a specified percentage or number?

I have an array object with a custom list of three objects (fire, water, air), currently it gives me a random 'value' within 23-32, how can each item on the list have its own range? Also how to get each value to increment up or down by specific percentage or number? Here is what I'm working with:
https://mockaroo.com/7c218440