I'd wanted to set a rectangle to 148*105mm. Easily done. However, when my screen zoom is set to 100% the rectangle is about 20% smaller when I put a ruler against my screen. I dare say I'm doing something foolish?!!

From the manual:

By default, a zoom of “1” corresponds to one Inkscape pixel per screen pixel. If you would like to calibrate the zoom to a real physical measurement unit (like inches or centimeters), you can do so under the Interface section of the Inkscape Preferences dialog. You will find there a slider that can be dragged to set a Zoom correction factor. Simply hold up your ruler to the screen and drag until the onscreen ruler matches.

http://tavmjong.free.fr/INKSCAPE/MANUAL ... -Zoom.html

Enter the resulting zoom factor into the zoom spinbox in the status bar to get 1:1 to the real world.

Note: This will not change how large the object is displayed at 100% zoom. It will only change what zoom pressing 1 on the numpad switches to.

Note that it's common for items viewed on a computer screen not to be the size they are said to be. There are all kinds of different computer screens and screen resolutions, and not to mention, different programs which use different native resolutions, which makes it impossible for someone to put an image on the internet, and expect it to look like the real size on every computer screen in the world.

But if you have the Scale set properly in Inkscape, according to the units you're using, then whatever you measure in Inkscape will be exactly that size - like for printing or cutting or plotting or whatever. Just not on the screen.