Frank,
Couple of things, first of all, if you have an MFT, you need a "Flag Stop" part number 495542. This slides along your fence and you tighten it down when it snugs up to your workpiece, already positioned for the cut. Then snug up all the rest of the cuts against your stop and every one will be exactly the same length. If you have a clean kerf on your mft, you can just measure the stop position from the kerf.
Secondly, we all have to decide our personal cutting style: leave the line, split the line, etc, the important thing is that whatever you decide - do it that way every time - you will save a lot of grief. With the track saw and the rail, its easiest to leave the line.
If you are doing a lot of cutting, google "story stick" for another technique for avoiding measuring errors.