Prawn - How to limit the text area on a form to match a fixed area on a Prawn PDF Report

We're building a Rails app which enables a user to make their own PDF booklet.

On the PDF document there are fixed height sections for the user to complete via a web form. For example lets say the user开发者_开发问答 has a half page 'About me' section at the bottom of the second page.

What's the best way to limit the text box on the form so that the user can only enter as much text as there is space available on the PDF output?

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Couldn't find an elegant way to do this with Prawn. Suggest just to add a javascript character count which the users sees as they type, like how twitter does it.