Tac wrote:
"My question is, for a given coax with 66% velocity propagation, is the
formula 66% of 492 (325/f) or 66% of 468 (309/f)?"

VF, velocity factor, is % of the free-space velocity of a radio wave
which is identical to that of light.

Velocity of light is about 300,000,000 m/sec.

Wavelength in free-space is:

WL= 300,000,000 / f meters

There are about 3.28 feet in a meter.

Polyethylene insulated flexible coax cables usually have a velocity of
propagation of about 66% that of free-space. Velocity depends on cable
dimensions as well as its insulating material and distribution.

In my ARRL Antenna Book, the length in feet for a 1/2-wave dipole is
given as 468/f (MHz).
As 1/2-wave= 150/f (MHz) x 3.28 ft/mtr=492/f (MHz), it is clear the
handbook formula has been discounted by 5% for "end effect", which is
actually a function of how fat the wire used is, and relates to wave
velocity reduction.

Best regards, Richard Harrison, KB5WZI