Chemfig - distance between arrow and Compound

Question

I would like to increase the distance between the arrow of the reaction and the compounds but when I change the value of arrow offset the direction of the arrow gets inverted. Also, when I change the value of compound sep it modifies the size of the arrow and I am not able to understand. As far as I know arrow offset should be the command to adjust the distance of the arrow from the compounds and compound sep thedistance among the compounds but I am not able to understand why they are not working as I expect.

Below I report my code:

\renewcommand * \printatom[1]{\small\ensuremath{\mathrm{#1}}}   
\vspace{1 cm}
\setchemfig{compound sep=6em,arrow offset=0.9em}
\begin{center}
\schemestart
\chemfig{[,0.75]R-[:30]N=[:0]C=[:0]O} + 
\chemfig{R'-[:30]OH}\arrow[0, 0.75]
\chemname{\chemfig{[,0.75]RO-[:30](-[:90,,,1]N(-[:150]H)(-[:30]R))=[:330]O}}{Urethane}
\schemestop
\end{center}
\bigskip
\renewcommand * \printatom[1]{\ensuremath{\mathrm{#1}}}

Thank you in advance for any your help!

Answer 1

\documentclass[10pt]{article}
\usepackage{chemfig}
\begin{document}
\renewcommand * \printatom[1]{\small\ensuremath{\mathrm{#1}}}   
\vspace{1 cm}
\setchemfig{compound sep=6em,arrow offset=0.9em,scheme debug=true}
\begin{center}
    \schemestart
    \chemfig{[,0.75]R-[:30]N=[:0]C=[:0]O} + 
    \chemfig{R'-[:30]OH}\arrow[0, 0.75]
    \chemname{\chemfig{[,0.75]RO-[:30](-[:90,,,1]N(-[:150]H)(-[:30]R))=[:330]O}}{Urethane}
    \schemestop
\end{center}
\bigskip
\renewcommand * \printatom[1]{\ensuremath{\mathrm{#1}}}
\end{document}

arrow offset=0.9em = 0.9x10pt=9pt=3.163mm

arrow length=0.75x6em-2x0.9em=2.7em=27pt=9,489mm

all distances are correct