Thank you GrannyGrump.
If I understand you correctly, you have found that it's a sort of best practice to use pt/px for hr? Considering how many very nice formatted books you have made, I will adopt that.
For this particular case, my guess is that there is a bug in the Sigil Previewer. I may be wrong, but that is the only place that does show the lines differently (compared to the ereaders and other software I tested with).
It doesn't matter much I guess, I just wanted to mention it since the amazing Sigil-duo seem to squash bugs in seconds.
(And if I did something wrong or "stupid" I hoped for feedback on that. Your post made my post worth it, thanks.
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