Question about tire pressure on winters

Hi all,

I live in Toronto, and we have had a cold spell for the last week. I have 18” steel winter tires (235/60R18), and one of the tires dropped below the threshold and set off the tire pressure alert. I noticed all the tires are about 30PSI (the lower one was at 28). What PSI should these tires be optimally set at?

EDIT: Thanks everyone for the feedback.