Tuning by hand: raise proportional gain until the loop just oscillates, then halve it.
The heart of the Ziegler–Nichols method. The oscillation point measures the process; half that gain buys stability margin.
Driving a real plant to sustained oscillation to find the limit is exactly what you must not do on anything with a person, a heater or a heavy load attached. And on a process with long dead time the oscillation builds slowly enough that you overshoot the limit badly before you see it.
Estimate with the rule, then check it against the calculator that models it properly.
Open PID Control →Tuning by hand: raise proportional gain until the loop just oscillates, then halve it. The heart of the Ziegler–Nichols method.