I use a dremel tool with a flat wheel burr or a cut off wheel. Cut off the top of the pipe above the floor with your regular saw so that it is level. Then use a pencil or grease pencil to mark a level line inside the pipe by holding the pencil and running your finger at the top of the pipe cut. Then use this line to run your dremel around on the inside of the pipe cutting to the outside. Be quick so that you minimize the melting and clean off the burrs.
It doesn't need to be exactly level and sometimes I do it in two dremel cuts if I can't get the pencil tip down the tube far enough. The first cut will get it below floor line, then the second will get it to just the right length. I can do a set of cuts in just a couple of minutes.