Hello again,
It seems that the threshold for the 'normal' CMOS gate could be 1/2 Vcc (which would be 7.5v at 15v Vcc) it's just the high and low definitions that are different. This must be so or else the linearized gate would not work. That is, when we place a resistor from output to input with an inverting gate of some kind, the gate is self biased into linear operation. That means there can be no hysteresis in a 'normal' gate, or at least one that is capable of linear operation.
The Schmitt Trigger on the other hand must have two distinct levels, so that makes it very different than the typical gate.
So in the formula for the resistors we can just make VL=VH and set that to about 1/2 Vcc and that should give us the right results. For the Schmitt Trigger however, we'd still have to set these two to two different values.
This actually makes it a little easier for normal gates.