I’ve been tacking an extra 15 minutes of fairly easy work, mostly just to burn a few extra calories and bump the volume a bit. I do it at the end to be sure the quality work is “in the bag,” because life happens and the extra 15 minutes might vanish with a phone call.
I usually end up doing around 55% or so. It’s a guess to get the target power kind of right, since you have to put in a factor relative to the ending target power of the cool-down prior to extending it, but I’m on a dumb trainer so this doesn’t prevent me from just pedalling a bit harder.