Many thanks John.
I think I've seen "scheduled" substituted by "regulated" performance in some discussions. This, as you've suggested, would imply a set of conditions which, if met, have been shown by some regulatory or certification authority to provide the required level of probability of a safe operation, be it a take-off, landing or whatever.
I'm fairly new to the field of aircraft performance and this was one of those innocent "why's that then?" questions that didn't seem to have an answer that I could find written down.
Thanks again
WUT