[TowerTalk] Carolina Winsome and vertical radiator question

[Bill Bennett]

I have noticed that the CW160 and CW80 have the same 21 foot vertical
radiator between baling and choke.  Why 21 feet?

Reason I ask is I am trying to roll my own CW160. I noticed that Radio
Works uses RG58 with foam dielectric and I would like to use RG8. Do I need
to account for velocity factor difference?
Thanks
Bill Bennett K4IBC