Here's how I save objects for import later. Note that there's gonna be more than one (good) way to do this...
First:
--In "Document Properties" -> "Grids", set the X and Y spacing to an even amount, like .00100 (rather than the default .00111) It's possible to set an "English" increment, like .125 or .0625...
-- Set inches as the default. I don't know how to do that globally, so it requires setting in more than one place (Doc Properties, etc).
-Turn on Grid.
-Draw the circle (edit the coordinates above if needed).
-Add the crosshairs. Grid is very helpful for both these steps.
-If there's any doubt about the symmetry--select everything (circle and xhairs) go to "Object" -> "Align and Distribute", then center everything on both the horizontal and vertical.
-Select all and make a group ("Object" -> "Group").
-Go to "Document Properties" and click on "Resize page to content", then "Resize page to drawing or selection", which will make the page the same size as the drawing.
-Save (as SVG vector file)
Then just "Import" the file when it's needed. The "Align and Distribute" tool is great for laying down multiple holes, assuming they are evenly distributed. Get the two ends correctly positioned and the tool does the rest...
Since Inkscape is a vector draw program (based on the PostScript language, not a CAD prog), the stroke widths can be a little confusing, but it's possible to select multiple objects and change the stroke. Tip: don't draw really tiny or really large objects. The stroke will change proportional to the resize.
There's really powerful scripting built into Inkscape, so that would be another way to create library objects.
Zip file 0.5'' hole with xhairs.