Ability to remember Screen Resolution Settings after update

Every time I update I have to mess around with the screen resolution settings to recreate what I had before. I have an iMac (Retina 5K, 27-inch, 2017) with a Dell P2715Q (5K) monitor which I use for my Parallels Windows 10 setup.
Sometimes it just accepts the old settings I specify the first time and away we go. Other times it's a rigmarole of settings change, reboot to save, settings change to something random, reboot, specify again, reboot, wait, wait and whew, this seems to have stuck.