Aside from all of the critical remarks, the way to determine the minimum gate voltage required is to use the data sheet curves. Certainly reading the data sheet is indeed tedious, BUT the actual product data sheets have a whole lot of useful information, that includes very useful graphs that show (usually), resistance, or conductance, versus the gate voltage, at different temperatures.
So my very serious answer is to download the manufacturer's data sheet pages.

As for that actual circuit, my question is what is the intended application?? And why that particular circuit??
It is a power supply of some kind, and my guess is that the goal is to provide some amount of current at some voltage.
It will help some of us to provide better answers if we know what the intention actually is.

Thank you

It is a triode tube tester. The circuit is not mine so I didn´t fell free about posting it

The goal was to make a test of the circuit with low voltage just to check doesn´t blow up, but maybe apliying very low voltage could be bad for the MOSFETs.

I have made the test first with half the voltage and later with nominal voltage and worked fine

 

as with any linear regulator, power dissipated by series transistor is proportional to current and voltage drop across that transistor. this drop is greatest when its output is set to low voltage...
suppose current is 50mA. suppose output is 22V. then transistor will need to endure (sweat as heat) power Pd=50mA*(422V-22V)=50mA*400V=20000mW = 20W. transistor can handle that with ease if heatsink is large enough.