If there's no minimum insertion indicator, I'd always go with the 30% rule. or 1/3 of the stem shaft inserted into the steerer tube, minimum.
I've encountered this ratio as a safe baseline in a lot of structural designs in buildings. Less woud usually require a special design to keep the safety factor adequate...
You might also consider going for a bit more insertion length if the stem forward extension is unusually long like maybe something over 120mm....
JMO

Chombi