\documentclass{article}
\usepackage[dvips]{web}

\def\r{\string\r}
\def\n{\string\n}

\begin{document}
\begin{Form}

\noindent First Sub total.

\medskip\noindent
\makebox[.75in][l]{Number 1:} \TextField[width=1in,name=firstnum,
keystroke={AFNumber_Keystroke(2,0,0,0,"",true);},
format={AFNumber_Format(2,0,0,0,"",true);}]{}

\medskip\noindent
\makebox[.75in][l]{Number 2:} \TextField[width=1in,name=secondnum,
keystroke={AFNumber_Keystroke(2,0,0,0,"",true);},
format={AFNumber_Format(2,0,0,0,"",true);}]{}

\medskip\noindent
\makebox[.75in][l]{Sub total:} \TextField[width=1in,name=total1,align={0 /_objdef {total1}},
format={AFNumber_Format(2,0,0,0,"",true);}, keystroke={AFNumber_Keystroke(2,0,0,0,"",true);},
calculate={AFSimple_Calculate("SUM",new Array("firstnum", "secondnum"));\string\040},
format={AFNumber_Format(2,0,0,0,"",true);}]{}

\bigskip\noindent Second Sub total:

\medskip\noindent
\makebox[.75in][l]{Number 1:} \TextField[width=1in,name=Number.1,
keystroke={AFNumber_Keystroke(2,0,0,0,"",true);},
format={AFNumber_Format(2,0,0,0,"",true);}]{}

\medskip\noindent
\makebox[.75in][l]{Number 2:} \TextField[width=1in,name=Number.2,
keystroke={AFNumber_Keystroke(2,0,0,0,"",true);},
format={AFNumber_Format(2,0,0,0,"",true);}]{}

\medskip\noindent
\makebox[.75in][l]{Total:} \TextField[width=1in,name=total2,
format={AFNumber_Format(2,0,0,0,"",true);},align={0 /_objdef {total2}},
keystroke={AFNumber_Keystroke(2,0,0,0,"",true);},
calculate={AFSimple_Calculate("SUM",new Array("Number"));\r},
format={AFNumber_Format(2,0,0,0,"",true);}]{}



\bigskip\noindent
\makebox[.75in][l]{Grand Total:} \TextField[width=1in,name=total,
format={AFNumber_Format(2,0,0,0,"",true);},align={0 /_objdef {total}},
keystroke={AFNumber_Keystroke(2,0,0,0,"",true);},
calculate={this.getField("total").value=this.getField("total1").value+%
   this.getField("total2").value;\r},
format={AFNumber_Format(2,0,0,0,"",true);}]{}


\special{! [ {corder} 0 [ {total1} {total2} {total} ] /PUTINTERVAL pdfmark}

\end{Form}
\end{document}