This code
\documentclass[10pt,a4paper]{article}
\usepackage[showframe,headheight=2in,headsep=0.1in,left=0.8in,right=0.8in,bottom=0.5in]{geometry}
\usepackage{xcolor}
\usepackage{hyperref}
\begin{document}
\begin{Form}
\noindent\textbf{DESCRIPTION OF ACTIVITY}\raisebox{-2pt}{\TextField[name=description,width=4.3in]{~}}
\end{Form}
\end{document}
gives the output

Notice that the text field box did not end at the right margin. This results to ugly alignment of the boxes after typing several lines with text fields. I could have used \hfill before the text field but that will leave ugly white space between DESCRIPTION... and the text field.
So, to my question: How can I define a width, remwidth, such that I can write
\noindent\textbf{DESCRIPTION OF ACTIVITY}\raisebox{-2pt}{\TextField[name=description,width=remwidth]{}}
so that the text field box stops exactly at the right margin rule?
I would appreciate it very much if the the answer would include an explanation of what is happening at every step.
Edit: October 15, 2012 I edited the question since the problem is really about the \TextField command and the hyperref package putting a padding in it as noted in the comments. The other part of my question on defining a width command that I can use elsewhere is actually solved by \linewidth, which I just learned last night even after 3 years of using LaTeX. The question Difference between \textwidth, \linewidth and \hsize clarified its use for me.

:-)– egreg Oct 14 '12 at 15:35\LayoutTextField, then\dimen0if fixed by \settowidth{\dimen0}{#2 \space}and you can get rid of the2.2221pt` that also might change depending on the glue of the text description. – Heiko Oberdiek Oct 15 '12 at 02:27\hboxwith your suggestion but the end of the text field will cross the right margin withshowframeoption ingeometrypackage. Why is that? – hpesoj626 Oct 15 '12 at 06:11hyperrefdrivers add a link margin of1pt:\addtolength\TextFieldLength{-1pt}. See also\refwith tight box surrounding the reference. – Heiko Oberdiek Oct 15 '12 at 11:34\kern1ptafter the text field that is shortened by\addtolength\TextFieldLength{-1pt}. – Heiko Oberdiek Oct 15 '12 at 11:58