I remember reading sometime ago about a minimum stack height for a carbon fork, but couldn't find what I read before. I know the max is 5cm, but what is the minimum?

I recently built up a new TT bike and used an EC90 SL fork on it. Using a Cane Creek Is6 headset with a Ritchey stem and no spacers under the stem the steer tube falls about 1-2mm short of the top of the stem. This is just enough clearance for the compression plug to work right.

The stack height of the headset top cover is about 1.5cm. Is that enough space between the bearing and the stem?