Well -velocity factor depends on the cable..............
It's 1/sqrt(Er) where Er is the dielectric constant of the insulating material.
so to quote a textbook:
Dielectric constants and velocity factors
of some common dielectric materials used in coax cables
Material Dielectric constant Velocity factor
Polyethylene 2.3 0.659
Foam polyethylene 1.3 - 1.6 0.88 - 0.79
Solid PTFE 2.07 0.695
what've you got ??
obviously the sleeve has to be cut to that length, but for the free part 0.95 is a good enough guess.
The length of the feed to the braid split doesn't matter - it's a transmission line. Measure from the end to the split point - and make the braid cut length fairly short - < 3mm would be fine at these frequencies.
Dipoles aren't that fussy.
regards
Allan
ps download a free copy of RFsim99 - written by an old colleague of mine. It's not HFSS, but handy for simple RF work...
A