Hence max gate charge was determined at 415Vds was to obtain Vgs capacitance to obtain peak current which apparently is 7.93nF for 15Vgs at specified Ids.Hi,
I'm not sure what your question is...
The output capacitance is acting as miller capacitance, thus ...
* Although the voltage at the gate changes only about 15V
* the voltage at the output capacitance may change 415V.
This causes high Q.