Mfcttrf

Yesterday I could swear the horizontal spacing between the end of the item-label (a number followed by dot) of a numbered-list and the item-text following that label is 5.2pt, although the output (showoutput log) shows 5.0. After running some tests today, it's (surprisingly) not 5.2pt but 5.05. Considering today's code was not a copy of yesterday's (which cannot be recovered by now, unfortunately), most likely explanation is changes in the code. However, 5.05pt is still not 5.0. That's one issue I would like a feedback from you about.

The second issue is that there's no length macro I know of that defines the spacing in question (between a list-label and its item) while such macros are usually available for adjustment (so I can't adjust the spacing of the list).

Now let's go through the following demonstration.

documentclass[varwidth]standalone
usepackagefontspec
setmainfontGeorgia
showoutput
begindocument
 beginenumerate
 itemhello
 endenumerate
 1. hello

 newlengthspaceWidth
 settowidthspaceWidth 
 thespaceWidth % 2.41211pt
enddocument

...........TU/Georgia(0)/m/n/10 1.
.........glue 5.0
........penalty 0
........TU/Georgia(0)/m/n/10 hello

........TU/Georgia(0)/m/n/10 1.
........glue 2.41211 plus 1.20605 minus 0.80403
........TU/Georgia(0)/m/n/10 hello

(Space of 2.41211pt is less than glue of 5.0pt.)

Replace 1. hello with 1.makebox[5.0pt]hello (and remove thespaceWidth):

beginenumerate
itemhello
endenumerate
1.makebox[5.0pt]hello

...........TU/Georgia(0)/m/n/10 1.
.........glue 5.0
........penalty 0
........TU/Georgia(0)/m/n/10 hello

........TU/Georgia(0)/m/n/10 1.
........hbox(0.0+0.0)x5.0, glue set 2.5fil
.........glue 0.0 plus 1.0fil minus 1.0fil
.........glue 0.0 plus 1.0fil minus 1.0fil
........TU/Georgia(0)/m/n/10 hello

Perfect (so far)!

Now remove setmainfontGeorgia:

Obviously, 5.0pt is NOT enough.

Let's try 5.05pt: replace 1.makebox[5.0pt]hello with 1.makebox[5.05pt]hello:

Perfect.

What's your take on it? Thank you.

In your document, both strings start at different horizontal positions. This difference does not account to a whole number of pixels on your screen, so you can't perfectly align the dots. So comparing the position on the h becomes meaningless. If you look closely at your last picture containing both hellos, you see that the red line does not touch both dots in the same way.

To get a more meaningful picture, you can ask TeX to indent your text in the same way it indents the label:

documentclass[varwidth]standalone
usepackagefontspec
showoutput
begindocument
 beginenumerate
 itemhello
 endenumerate
 settowidthleftskip1. % Don't do this in a real document!
 advanceleftskip -20pt
 leftskip-leftskip
 noindent1.makebox[5.0pt]hello
enddocument

This moves the 1. hello to the right to align with the label list entry. Now both are perfectly aligned, both the 1. and the hello. So the distance actually is exactly 5pt.