This is how you do customer support

Are you using the Tabbed Box Maker? Here is the github page:

There are several box maker extensions and sometimes when I have installed Inkscape, I forget which one is which. I use the millimeters because the imperial one allows only two doesn’t got to three significant digits and you need that for precision in tight fitting. The only time you need to convert is thickness for the focus of lid camera, otherwise there is no need (other than knowing how big a part can be in metric)/.