No equations to remember - just rules of thumb. The higher the Z, the lower the C. I don't recall the exact numbers, but I think 50 ohm cable is about 30 pF/foot, while 75 ohm is about 25 or so, and so on, as Z increases. there are all kinds of data and tables and such with precise numbers, but the trend is that Z mostly determines C (and L too), regardless of dielectric constant and velocity of the insulators and such.
Ed
