Here's a handy, easy to use tool:
umm-usa.com/.../popup_image.php
UMM is also a good source for thin razor saws, which I sometimes find easier to control for straight lines.
I've used other scribing tools as well; #11 blade, sewing needles, a Squadron scribing tool, etc. all with equally amateurish results, and like you, I'm looking for a better guide tape that the Dymo brand. I can still get it, but I find it to be thick, and would like to get something more flexible. Some have suggested automotive pinstripe tape, so I may give that a try.
For me, 2015 will be the year I intend to master scribing. Happy New Year!
Mike