I use this:
pi@MagicPi:~ $ grep rotate /boot/config.txt
display_hdmi_rotate=3
The number can be different depending on how you turn it.
I have the bottom on the left side, if you understand what I mean.
Turned it clockwise.
When turning, take care of the viewing angle.
My monitor has a rather poor viewing angle, especially from the bottom, so now on my left.
I need to look at the Mirror frontal or from the right (not too much though).
But luckily this setup works for me.
I did not purchase a IPS panel, as it uses more power than regular TFT, having it turned on for several hours a day.
There was a new thread in the last days, explaining how to do it via css, but personally did not try it yet.