you can combine that...
have a layer and update the % display value in the viewchanged event
in the mouse event you only control fading in or out the layer
edit:
in the mousewheel event you would have to set a variable, maybe with a time stamp
and in a separately running renderloop you'd have to constantly check the variable,
and if it didnt change for some time, fade the layer out.
possible but quite complex for a fov % value