It's tough enough to cut a straight line freehand in something flat. It's really hard in something round. So to make accurate cuts in Sonotube concrete forms, I guide the saw with a strip of Sonotube. As shown in the drawing, I first cut a strip approximately 3 in. wide off the Sonotube, which gives me a factory edge that I know to be square. I then cut a couple of slots about 6 in. from one another (this cut depends on the size of the hose clamp -- I use a 4-in. clamp for this application). Next I make a cut through the strip between the slots and thread the hose clamp through the slots.
To use the guide, I slip it over the form to be cut, and then I tighten the hose clamp until the guide is snug. The factory edge of the guide should be offset from the cutline to allow for the distance between the sawblade and the edge of the saw's foot.
Rick Tyrell, Prosperity, SC