I want to make a flexible, portable, vertical 10M dipole from coax by
folding back roughy 1/4 wavelength of the sheath and taping it in
place.

I would not expect the sheath to perform like an equal length of solid
or stranded wire because if the extra length of wire. Is there any
theoretical way to compute a factor, or do I just keep snipping until
I get lowest SWR at the desired freq?

Ken KC2JDY