The inter-winding capacitance of a 150 or 500 turn tightly wound coil is insignificant compared to 310pF?
I very much doubt it. The self-resonant frequency of the coil will certainly be affected by the FET's loading
and the secondary winding, but the details of the coil's self-capacity are very important - its very complicated
to calculate all that - either measure the actual inductance directly from V dt = L dI or calculate it from a good
equation for that geometry of coil.