Hello again,
It dawned on me that it may be simpler for you to measure the gate charge requirement for your application than to actually try to calculate it.
You'd have to set up your device to switch with the required drain voltage and load, and measure the gate to source voltage or maybe the drain voltage. If the time it takes to switch the drain voltage to near zero is t1 and we start applying a gate current Ig at time t0, then the total gate charge is:
Q=(t1-t0)*Ig
When looking at the gate voltage, it will first ramp up past Vth (due to the gate source capacitance) then it will level off (due to the gate to drain capacitance and falling drain voltage) then later it will start to ramp up again. The time t1 above will then be the time when it starts to ramp up for the second time.
So the drain voltage falls more or less as a ramp, and the gate voltage rises as a ramp, then flattens out, then rises as a ramp a second time. Either the start of the second gate ramp or the end of the drain ramp is the time we call t1.
Either of these might be the best way unless you can get a spice model and try to get the right values for that model too.