The issue being that if you deliver a Scalp Cut into someone waiting in Fool then a likely outcome is they will deliver a false edge cut to your hands with a step backwards. This doesn't to my mind preclude the Scalp Cut as very useful against Fool but only highlights the general foolishness of attacking a prepared position against a opponent who is trained in the same technique and aware of basic geometry:
1. If you are in Fool expect a Scalp Cut
2. If you aim for his head with a Scalp Cut and he gathers backward then his sword will be in measure to hit your hands and your sword will not be in measure to reach his head.
Of course you might also reach his hands at the same time, he might miss your hands and then he is lined up for you to deliver a beautiful thrust etc. However, there's a good chance he's getting a hand hit and if he keeps retreating out of measure he'll probably get to restart without any negative outcomes.
I use a Scalp Cut to the head regularly against Alber (most point off line positions in fact) and it's effective. It's about provoking a time to strike or about recognising a situation where it is likely to succeed..
These are the "times" and provocations I think I already use:
1. My opponent cuts through into a Fool position, I have slipped back with his cut so while he over cuts I step forward again and tag him straight down on the head (After)
2. I go into Day and as my opponent responds by changing down to Fool, as soon as his point comes off line I spring in and hit him before he settles in guard
3. My opponent is in Fool, I gather and drop from Day as if I'm going to block their sword. As they pause to think about moving out from under my block I strike them to the head
4. I quickly step to one side with my rear foot, if they automatically step to realign I attack (this one is tricky and almost has to be executed as one move)
5. I gather my front foot backwards, if they automatically step forwards a full step I attack
These are the situation judgments I would assault a prepared Fool with a Scalp cut:
1. If my opponent was "timid" and I would expect them to balk rather than respond aggressively
2. If my opponent was especially slow
3. If my opponent was using an especially shorter sword
4. If my opponent was especially smaller in stature
Or you can just do what I feel Ringeck is suggesting which is basically to not throw it at the head but to provoke halfsword:
Throw the Scalp Cut short (either out of measure or with withdrawn arms would be my thought) to threaten the face with a thrust, he'll engage your blade to take the point off line and come to halfsword. Perfect, let the halfsword commence.