AK wrote:
Is 78% the correct velocity factor for RG8X coax?

NO.

RG-8X is really not an RG cable, and has no manufacturing standard.

Foamed cables can vary from near .7 vF all the way to near .9 vF

The manufacturer normally adjusts the ratio of air to dielectric to get
the correct impedance. Since RG8X has no standard dimensions or
conductor diameters, and it has no standard impedance range, the vF
will vary all over the place.

RG8X is normally a much denser foam (less air) than other foam cables.
The RG8X cable I have is in the low .7 range as measured on a network
analyzer. I've never found any with higher than the upper .7 range.

Since there is no manufacturing standard (meaning it is NOT really
suitable for an RG number) you should measure the cable.

73 Tom