How to use expression to set margins using \hoffset in TeX?

Question

\def\leftmargin{1.25in}          % custom margin
\hoffset \leftmargin - 1in       % subtract 1in from \leftmargin, !Not working
\hoffset \leftmargin minus1in    % also tried this with no success


% In effect, \hoffset should be 0.25in

How do I use simple expression while setting dimensions in Plain TeX?

Answer 1

With Knuth's original TeX (tex):

\newdimen\leftmargin
\leftmargin=1.25in
\hoffset=\leftmargin
\advance\hoffset by -1in
\showthe\hoffset

With e-TeX (etex, pdftex, xetex, luatex):

\def\leftmargin{1.25in}
\hoffset=\dimexpr\leftmargin - 1in
\showthe\hoffset

Both give an output of

> 18.0675pt.

which is the same as 0.25in.

Answer 2

With OpTeX:

\hoffset=1.25in

That is all. The left margin will be 1.25in after such setting, because OpTeX removes the "1in confusion" and "wasted calculation", because it sets the original point (0,0) to the upper-left corner of the paper sheet directly.