Instead of masking tape I use the heavy cardstock that those mail advertisements for windows, Republicans, Democrats, whatever, that come in the mail. Wrap tightly around the tube to be cut, tape in place. It usually takes several wraps (the more the better). That provides a stouter guide against which the knife blade can rest. If the blade cuts into the guide that's usually not a problem because the guide is so thick, the blade doesn't cut through all the layers of the guide.
Sand afterward, checking as to whether the cut is square with a good carpenter's square.w
Best,
Terry