40mm when I looked seemed to be the standard stack.
30mm seemed to be about the lowest and most you can get away with 5mm less, some will go a bit lower (probably not such a good idea in the 300mm ones)
FSA did some new 30mm versions, don't know if they still do. Their general ones are 40mm but they are all new.
Syncros stems I can check for you, but 40 to 45mm but will clamp a bit lower.
I have one in my head that should be low, multi colours, often purple on turquoise... cinch clamp... expensive retro stuff..... Ringle but forget the name of it.. off to make coffee.
check your headset to, what are you using ?