Ditto what Hollice said ... adjust your needle position and run as many tests as you need to. Test with 3 strips of the same fabric you are sewing then measure the outcome.
Would like to add that different types of fabric can also make a difference. A Hoffman Asian print for example is considerably thicker than a batik. when you press your seam to one side, the Asian is going to take up more "space". The smaller the piece, the more it will make a difference.
Lastly ... I just recently started using the "vintage" guide slide. I use it because I am sewing very narrow strips and it's hard to hold them on the left side of the needle when I had a fatter foot on the machine (my 1/4" guide is a wide foot). So I switched to a narrow foot and I use the slide. I don't know why I didn't do this AGES ago as the guide is longer to the front of the foot which gives me more time/space to properly guide the fabric. I no longer get wider or narrower starts and stops.