Thank you Romanator, for the link and subsequent follow-up.
I apologize for not replying sooner, but celebrating my wife’s birthday got in the way.
When I first looked at the ArchWiki, I thought the the 2 steps were 2 different options of achieving the same goal, so I tried the force font dpi with the scaling back at 1 (since it hadn’t effected any change). Unfortunately, that didn’t work either.
It was only after I though about it a bit more that I decided to try both scaling AND force font dpi. I set the font dpi to 130 and the scaling to 1.4 (a rough approximation of the ratio of 96:130). That helped quite a bit.
I then brought up Kruler and set the cursor at 130 pixels and laid a scale against the screen. I measured a bit under 1". Adjusting the scaling to 1.5 got 130 pixels even closer to an inch, so I’m on the right track.
A bit more fiddling around, and I’m confident that what I see on the screen will be a very close approximation to reality.
I realize that I can adjust the font size in Firefox and Thunderbird, but at least in Thunderbird that only affects the font size of outgoing mail. It also does nothing for LibreOffice, WireShark, etc.
Curiously, the X-Server tab on KInfoCenter still shows a 96dpi screen of 423 x 238 mm (the actual dimensions are more like 309 x 174 mm). I assume that this is caused by poor communication between X-Server and KScreen2, due to a bug in one or the other (or both). Is this worthy of a bug report?
At any rate, thank you for pointing me in the right direction. it is very much appreciated.
My external monitor should be 1900x1200 on 17" but I can only activate 1600x1050. So everything is to big and would like higher dpi on the external monitor. The dpi for the notebook screen is 1920x1200@15"
Is there a way to adjust the dpi for the external monitor?
Kscreen is running great in KDE4. I would keep my eye on kscreen in Plasma 5. If it’s giving your grief, try renaming .local/share/kscreen and reboot.
I highly encourage you to submit a bug report if you keep encountering problems. The sooner we come across these issues the sooner we can get them patched.
Keep a notepad next to you when you are making changes to keep track