Concrete form tubes make the installation of footings for decks much easier, but cutting these tubes level can be an awkward process. Here's a simple tip that will make cutting straight lines on a curved object a cinch.
Cardboard concrete form tubes are noulds that you pour concrete onto for a perfectly round footing. It's vital that your tubes are cut level across the top and the bottom to ensure an even foundation.
The video linked below shows you how to make a jig by cutting a 3" portion of the cardboard from the factory edge, then using a hose clamp to expand and contract the jig around the tube. Use the factory edge that is straight as an arrow (put a level on it to be certain) and make your marks.
Straight Cuts on Cardboard Concrete Forms [Fine Homebuilding]