Glad you've still got those teeth.
Here's what I did. It's a combination of many different suggestions I found while searching around.
1. Remove the topcap and expander and any of the spacers you had on top.
2. Measure from the top of the steer tube to the top of the stem (the length of the steer tube sticking out).
3. Remove the stem & remaining spacers and disassemble the rest of the headset. Make sure to keep track of which bearings go where and which way is up!
4. Measure down from the end of the steer tube the distance you got in step 2 and
add 2mm to that length. Wrap some masking tape tightly around the steer tube so that it's about centered on the spot you measured. Measure again and mark on the masking tape where you're going to cut.
5. Attach a hose clamp (or pipe saw guide thing if you've got one) so its top edge is sitting right on the line you marked. This will be your cutting guide. If it looks like the hose clamp will be against the carbon, add an extra wrap below the tape you've already applied so the hose clamp will not scratch your steer tube.
6. Use a fine toothed hacksaw (32tpi is what I used, I wouldn't recommend going any coarser) and cut along the top side of the hose clamp. I used some oil on the blade to keep from making a lot of dust. You don't want to be breathing carbon fibers. I've heard others have used water instead of oil. I'm not sure it matters since you can clean it when you're done anyways. Don't be in too much of a hurry to cut through it too fast; take small bites and take your time.
7. Once the end it cut off, remove the hose clamp and masking tape. Use some fine grit sand paper to polish the end of the steer tube and make a small chamfer on the edge to remove burrs and avoid cuts. Don't use too wide of an angle; the chamfer should go down no greater than 1mm from the end.
8. Clean up the steer tube and reassemble the headset, spacers and stem. Verify that the steer tube ends no deeper than 3mm from the top of the stem. Then reattach the expander and top cap.
Hope that helps. Everything went really smoothly for me; I had no issues at all and it was a very easy job.
I know it's been posted here before but here's cannondale's instructions for the CAAD10 expander and top cap assembly.
