Principal parameters of cores are

Bmax (Maximum flux density)

Crossection Area

Window (space for the coils and insulation)

Al (relates thepermeability of the core to the geometry, determines the inductance a coil will have for a number of turns of wire squared)

Gap

Magnetic path lenght

to determine the Al of a core make a coil with n turns measure the inductance and divide the measured value by n squared and you get the value of Al good to use for your measurement. Gap should be lowest or zero ... for toroids spread the turns over all the toroid as the gap is distributed in it... the more turns more precision.. but not too many turns are required... 10 or so is ok depending on core size... Al is normaly measured in nanoHenries or microhenries... the nucleus i have are 3000nH per turn squared

To find the Bmax of the core design a numbe of turns for a predetermined lower bmax say 0,100T and vary the voltage to see where the core saturates... getting the voltage where it saturated back to the equation will equate for the correct bmax of the core...

Common knowledge say Bmax of ferrite is 0,3T and silicon iron laminates 1,5T

Window will tell you the turns you can get into the core... for high voltage insulation a winding factor of 50% is assumed for some space is required for insulation... from this you must aproximate from the 50% remain space to the maximum number of turns you can get for a particular wire diameter... assume the wire to be square so you dont get fooled by empty space among round wires... so the window remain in mm2 divided by the wire diameter also square so in mm2... result is the number of turns.

When working with a core the lenth of the wire becomes less important than the turn numbers to get similar voltages is simple as getting the same number of turns regardless of the difference in diameter of the layer of the coil. Because the core closes the Bfluxaround the all coil.