Per Cannondale user's manual:

The steerer tube length for any Cannondale carbonfiber fork with a carbon steerer is limited to theMAXIMUM STACK HEIGHT of 55mm. This affectsthe height adjustment of the stem. No spacers maybe used on top of a stem. The stack measurementmethod is shown on the following page. If thesteerer tube is cut to accommodate a lesser spacerstack height, please observe the following:

The following procedure should only be completed by a professional bike mechanic.
1. Assemble the fork, headset, spacers, and stem without tightening the stem bolts onto the head tube.When the system is assembled, the carbon steerer tube should be 2-3 mm below the top stem. All spacersmust be located below the stem and within the maximum stack height as shown. No spacers may beused above the stem.
2. Set-up the compression assembly before inserting it. Adjust the length so that the expander is located atlower stem bolt. The top cap and the expander end provides critical support to the carbon steerer whentightening the stem bolts. Adjust the length by threading the top cap on the expander parts.
3. When the assembly is the correct length, insert it into the steerer tube. It is designed to fit snugly insidethe steerer. Insert an Allen key through the access hole in the TOP CAP and into the EXPANDER BOLT.Tighten the to the specified torque.
4. Now, to set bearing preload, insert a 6mm allen key into the hex shape in the TOP CAP itself. Turn theentire top cap clockwise to increase preload. Turning it counter-clockwise will decrease the preload. Whenthe headset preload feels correct, turn the stem to align the handlebar and tighten the stem fork clampbolts to the torque specified for the stem. Consult the stem manufacturer’s instructions. The torquevalues for components are often marked on the part.