How do I draw a margin that also is attached to a short rule?

Inside the provided myproof environment, use z for a vertical bar, z* for the vertical bar with the horizontal foot. The optional argument to myproof provides the indentation for reason.

Because I redefine par in the environment, you must use exactly one blank line between proof steps.

documentclass{article}
usepackage{tabto}
defzz{1.8dimexpr}
makeatletter
newcommandz{@ifstar{zstar}{znostar}}
makeatother
newcommandzstar{makebox[1em][l]{%
  smash{rule[-dimexpr2pt+zzdpstrutbox]{.5pt}{zzbaselineskip}%
    rlap{rule[-zz.9dpstrutbox]{14pt}{.5pt}}}}ignorespaces}
newcommandznostar{makebox[1em][l]{%
  smash{rule[-dimexpr2pt+zzdpstrutbox]{.5pt}{zzbaselineskip}}}%
  ignorespaces}
letsvparpar
newcounter{proofline}
newenvironment{myproof}[1][1.8in]{%
   setcounter{proofline}{0}%
   parskip0.8baselineskip
   defreason{tabto{#1}}%
   defpar{stepcounter{proofline}%
   svparnoindentmakebox[1.5em][l]{theproofline}}%
   parignorespaces
  }
  {letparsvpar
   parnoindent
   ignorespacesafterend
  }
begin{document}
noindent Before
begin{myproof}
z* $(Prightarrow(Qwedge R))$ reason Premise

zz* $P$ reason Assumption

zz $(Qwedge R)$ reason 1,2 $rightarrow$E

zz $R$ reason 3 $wedge$E

z $(Prightarrow R)$ reason 2-4 $rightarrow$I
end{myproof}
After

noindent parskip was restored
end{document}