Regarding a psik reisha, my earlier question focused on whether the resultant action is judged according to a standard, but I am also unclear about what determines the "inevitable" nature of the outcome.
If you cut the head off of a chicken, it will most probably die. If you drag a heavy bench through soft ground, it will make a furrow. But through less hard ground, the act might not lead to an inevitable furrowing melacha.
How does one determine the line between "it will" and "it might not"?
To wit, I received an email with halachot (Dirshu, 8 Nisan) which reads
A person may not comb his hair on Shabbos because it is a psik reisha (inevitable) that hair will be removed. It is even forbidden to use a comb to move hairs into place without really combing them, because hairs also get pulled out that way.
The followup halacha reads
A soft brush may be used on Shabbos if it is not a psik reisha that hair will be removed. If it is inevitable that hairs will be removed by brushing, the brush should not be used for regular brushing but may be used to align the hair.
I comb my hair with a comb every day and have precious little hair left so I stay very conscious every thing that gets pulled out, and my comb doesn't do the damage. How was it determined that the comb is a priori invalid -- what measure of inevitability was satisfied? Is it a personal standard (may I use a comb whereas someone else can't) or is universally derived and applied?