All the factors you mentioned affect the amount of flare, including surface and core shape.
Layout will also dramatically affect the amount of flare, with pin postioned at 3 3/8" from your axis flaring the most, on a symmetric core, further away will flare less with more length, closer will flare less with less length.
An asymmetric core will retain more flare than a symmetric core when moved further away from 3 3/8" away.
Extra holes and their placement can also somewhat increase or decrease flare.
Hope this helps