parbox automatic width calculation

Consider the following MWE:

documentclass[letterpaper]{article}
usepackage{amsmath}
begin{document}
noindent
$begin{aligned}[t]
  left( frac{1}{2}-frac{1}{4} right) left( frac{1}{2}-frac{3}{4} right) 
  &= left( frac{4-2}{8} right) left( frac{4-6}{8} right) 
  & & parbox[t]{0.5textwidth}{Simplify within parenthesis. 
          Simplify within parenthesis. Simplify within parenthesis. 
          Simplify within parenthesis.} 
  &= left( frac{2}{8} right) left( frac{-2}{8} right) 
  & &parbox[t]{0.5textwidth}{Add within grouping symbols.} 
  &= left( frac{1}{4} right) left(-frac{1}{4} right) 
  & & parbox[t]{0.5textwidth}{Multiply.}
  &=  -frac{1}{16}
end{aligned}$
end{document}

issues the error

This is primarily due to the parbox[t]{0.5textwidth}{<content>}.

Is it possible to create an environment that calculates the remaining linewidth for each line independently that the parbox can use?

---

Consider the MWE using the linegoal package suggested by @zarko:

documentclass[letterpaper]{article}
usepackage{geometry}
usepackage{mathtools}
usepackage{linegoal}

%---------------- show page layout. don't use in a real document!
usepackage{showframe}
renewcommandShowFrameLinethickness{0.15pt}
renewcommand*ShowFrameColor{color{red}}
%---------------------------------------------------------------%

begin{document}

    begin{alignat*}{2}    
left( frac{1}{2} - frac{1}{4} right) left( frac{1}{2} - frac{3}{4} right)
    & = left( frac{4 - 2}{8} right) left( frac{4 - 6}{8} right) 
        &qquad     
            &   rlap{parbox{linegoal}{raggedright
                Simplify within parenthesis. Simplify within parenthesis.}}
                                                                    [1ex]
    & = left( frac{2}{8} right) left( frac{-2}{8} right)        
        &   &  rlap{parbox{linegoal}{raggedright
                Add within grouping symbols}}                       
    & = left( frac{1}{4} right) left(-frac{1}{4} right)         
        &   & rlap{parbox{linegoal}{Multiply}}                   
    & = -frac{1}{16}
    end{alignat*}
        begin{alignat*}{2}    
left( frac{1}{2} - frac{1}{4} right) left( frac{1}{2} - frac{3}{4} right)
    & = left( frac{4 - 2}{8} right) left( frac{4 - 6}{8} right) 
        &qquad     
            &   rlap{parbox{linegoal}{raggedright
                Simplify within parenthesis. Simplify within parenthesis.}}
                                                                    [1ex]
    & = left( frac{2}{8} right) left( frac{-2}{8} right)        
        &   &  rlap{parbox{linegoal}{raggedright
                Add within grouping symbols}}                       
    & = left( frac{1}{4} right) left(-frac{1}{4} right)         
        &   & rlap{parbox{linegoal}{Multiply}}                   
    & = -frac{1}{16}
    end{alignat*}
end{document}

it issues the warnings shown below:

Is there a fix to this?

documentclass[letterpaper]{article}
usepackage{geometry}
usepackage{nccmath}  
usepackage{linegoal}

%---------------- show page layout. don't use in a real document!
usepackage{showframe}
renewcommandShowFrameLinethickness{0.15pt}
renewcommand*ShowFrameColor{color{red}}
%---------------------------------------------------------------%

begin{document}

begin{fleqn}[4em]
[
begin{aligned}
left( frac{1}{2} - frac{1}{4} right) left( frac{1}{2} - frac{3}{4} right)
    & = left( frac{4 - 2}{8} right) left( frac{4 - 6}{8} right) 
        &qquad     
            &   rlap{parbox{linegoal}{raggedright
                Simplify within parenthesis. Simplify within parenthesis.}}
                                                                    [1ex]
    & = left( frac{2}{8} right) left( frac{-2}{8} right)        
        &   &  rlap{parbox{linegoal}{raggedright
                Add within grouping symbols}}                       
    & = left( frac{1}{4} right) left(-frac{1}{4} right)         
        &   & rlap{parbox{linegoal}{Multiply}}                   
    & = -frac{1}{16}
    end{aligned}
]
end{fleqn}

end{document}

documentclass{article}

usepackage{tabularx}
usepackage{amsmath}

begin{document}


begin{flushleft}
  begin{tabularx}{linewidth}{
@{}
>{$displaystyle}r<{$}
@{}
>{$displaystyle{}}l<{$}
>{raggedrightarraybackslash}X
@{}
}
  left( frac{1}{2}-frac{1}{4} right) left( frac{1}{2}-frac{3}{4} right) 
  &= left( frac{4-2}{8} right) left( frac{4-6}{8} right) 
  & Simplify within parenthesis. 
          Simplify within parenthesis. Simplify within parenthesis. 
          Simplify within parenthesis. [12pt]
  &= left( frac{2}{8} right) left( frac{-2}{8} right) 
  & Add within grouping symbols. [12pt]
  &= left( frac{1}{4} right) left(-frac{1}{4} right) 
  & Multiply.[12pt]
  &=  -frac{1}{16}
  end{tabularx}
end{flushleft}

end{document}