The information regarding 2-adjacent-wire characteristic impedance - on Stack Exchange says :
"Yes, wire diameter matters. Assuming the two wires are round and of equal diameter, the impedance is (120/√ϵr) * acosh(D/d), where ϵr is the relative permittivity (dielectric constant) of the material between the wires, D is the distance between wire centers, and d is the diameter of the wires.
In cases where the spacing is significantly greater than the diameter, you can use the simpler formula (120/√ϵr)*ln(2D/d). This has less than 1% error when D/d is more than 3.6.
The velocity factor is simply (1/√ϵr), as user10489 points out.
Estimating ϵr can be tricky; for ladder line you need to figure something based on the material and the "fill factor" (how much of the space between the wires is occupied by plastic and how much of it is air). The goal of "open wire" feedline is to reduce this fill factor to nearly 0, which makes ϵr
decrease to nearly 1, and VF increase to nearly 1.
The dielectric constant of polyethylene is around 2.3; the ϵr of ladder lines made with PE webbing is between around 1.2 and 1.55, based on published velocity factors between 0.8 and 0.91."