Consider the following example:
\newlength{\textsize}
\makeatletter
\setlength{\textsize}{\f@size pt}
\makeatother
\setlength{\parskip}{12pt plus 1pt} % WORKS
\setlength{\parskip}{\textsize plus 1pt} % FAIL
How to make the failing line work?
Which means: how to set a length to another length plus and/or minus other values?