I think that taking a look at the Wikipedia article about PID controllers may help you understand better.
The SSR is NOT a proportional device - it is like a binary switch which is either on or off. Once the on threshold voltage is reached on an SSR, on the control side, the switched side, which is switching the 120 VAC to your heater is on. Period. What is proportional is the TIME that the SSR stays on, and that is controlled by taking into account past, present and probable future deviations of the output from the set value. So, what you as a user will notice is that well below the set point temperature, the SSR switches ON the heater. AS it gets closer to the set point temp, it starts to pulse the heater, on-off-on-off and so on. When the set point is reached, it may continue to pulse depending on various factors and settings. For example, there is hysteresis, which is an intentional zone of delay in temperature control, so that the SSR doesn't "bounce" on and off rapidly. YOur best bet is to set the desired temperature, and press the button to turn on the learning mode, and let the controller decide how best to control the setup- it will account for past, present and likely future errors depending on how fast the heater heats up, the thermal mass of the heated mass and so on, and will likely come up with the best settings.
One other thing - i had a MyPin pid and type K TC, in which the displayed temp drifted significantly. I wrote to the manufacturer's rep via AliBaba, and they indicated that it was not an unexpected thing with K thermocouples. Her advice, to rule out the PID itself, was to short the input for the thermocouple with a piece of wire. That should produce a steady temp display. I haven't tried it, as the vendor i purchased from via
Amazon took the problem as a PID problem, and refunded my money, and I purchased a different unit which is more steady. But for anyone reading this thread who is experiencing drifting temperature display, say, in an ice-water bath, try the trick of shorting the TC terminals on the PID to see if it is the PID or the TC.
bert