The issue is the method you are using. Measuring, marking and placing the track each result in small amounts of error that add up. When you need something repeatable, look for a way to create fixed stops that take the repeated measuring and marking out of the equation. So for example, if you were crosscutting pieces on a table saw to 24", you could set up a stop block clamped to the fence, slide your piece up against that stop block, then make the cut. You can do this all day long and each piece will be consistent.
Same principle applies to the guide rail. That's how the parallel guides work (both festool and 3rd party). But even without them, you can create your own stop block with a hook on one end to fit over the work piece butt the track against the other end. It's best to have the hook designed like a T so the top part that hooks is 90 degrees the part that butts up against the rail. This avoids any error caused by skewing the stop block. Line up each end of the track using this block and check it a few times until the guide rail is set perfectly. Use clamps to prevent the guide rail from moving and make your cut. This works well if the finished piece you are ripping is wider than the track. If it's narrower, you need to flip your stop block upside down so one end hooks on the rail and the other butts up against the work piece. Make sure you add additional support for the guide rail if you're ripping a narrow piece.
