right time to set the TUN_ST to 1

I want to know what is the right time to set theTUN_ST to 1, as I also use this block for the temperature control for a heater exchanger today, but I cannot switch to the phase 2. From the attached picture, the malipulated varialbe should be changed >5%, but today I don't try it, any other person have some experience for this?

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New subject after splitting from CPU312 & PID Loops?