What is the best way to bump a calculation value up to a minimum charge?

Minimum charge would be $20 - How would I bump $16.50 up to $20?

Thanks

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.