How to make the Windows 10 lock screen / slideshow / screen off gettimed correctly
 

Windows 10 has settings in about 5 different places for when your screen should switch off, when it locks, when you get a screensaver etc.
All I wanted was after X minutes, for a slideshow of pictures to play, then after Y more minutes, the screen turns off.
The lockscreen has a built in slideshow, but timing it is very difficult. After much experimentation, I've found the following method to work:

To set the two times:
X = time from no user activity to screen locking and immediately slideshow beginning
Y = time the slideshow plays for before switching off the screen

Do these in the following order and close each settings page before going to the next instruction, as things reset other things:
Start menu, "lock settings", screen timeout, set to X
Start menu, "lock settings", screen saver, set to none and X minutes, and on resume display logon screen
Start menu, "lock settings", select slideshow and choose folder of pictures (including subfolders), advanced settings as required
Regedit, HKEY_CURRENT_USER\Software\Microsoft\Windows\Curre ntVersion\Lock Screen, key "SlideshowDuration" (create 32bit Dword if not already there), set to number of milliseconds for Y