_jis_Yeah, I use Nova too, but I don't use FP to unlock so the only functionality that I have found with Nova or Tasker is to lock the display, which I would prefer not to do. I really just wanna turn off display. So sometimes after failing with the double tap twice I just push the power button. Thanks for the suggestion, though.
Exactly, it is not only you, I can confirm this. Fortunately, you can use this gesture above any empty place on the homescreen.
I use the Nova launcher myself. I used these two gestures for that: two finger rotation clockwise and anticlockwise, both of which I set to the screen lock. These gestures work seamlessly anywhere on the home screen (above any icons or widgets, no problem).