Set value maximum on numeric fields?

Hi all,

My first time posting a question and I'm stumped. Can anyone help? Image may be NSFW.
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Would anyone know how to set a numeric field to display any number UP TO a maximum that the form designer pre-determines? I've attached a screen-shot of what we're trying to do.

E.g. User enters rent ($500) x number of months (8) = $4000 (fartherest right field, in the picture below). I've figured out how to make the fields calculate, but is there a way

to set a maximum number that can be displayed? The idea is that people are filling out a budget, but we have established maximums that they can claim...i.e. $3000 for rent.

Thanks SO much for any help you can offer!

Image may be NSFW.
Clik here to view.