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I'm not quite sure if I understand your concern correctly. But from what I am visualizing, you want to change the current minimum value based on a user input, is that correct?
If that's the case, instead of changing the value, you might need to set separate number fields with different minimum charges.
I will then use conditional logic to show those number fields with specific minimum values based on the calculation done. Here's how my conditions are setup:
To learn more about conditional logic, you can check this guide: http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic
Here's a demo form with the changes I made above: https://form.jotform.com/53260685886973?
Let us know if that helps.