Yes the folding rule just demonstrates that you can slide the fence to a certain position, and shure its better to fix a measuring tape.
But the best way to get an exact parallel left and right fence, is to use two wooden sticks which you can slide and fix with bolts to the distance you need. This stick should be placed between the slideable fence and the guiderail, so you have to add the width of the guiderail for the exact measurement.
When you use the same stick to adjust the right and the left fence, you can be absolutley sure that the cut is parallel. The most important part when you cut cabinet sides is „parallelism“ and not the perfect half-millimeter-precision width of the cut.
Nobody will find out that a cabinet is one millimeter to small, but can see directly when something isnt perfect parallel.
I often tell my students that they should not stick to much to the measurements in a woodworking plan, because this is a more theoretically cut list depending on certain thicknesses of sheet goods. But in real a 19 mm thick board could vary from 18,5 to 19,5 mm.
A good woodworker doesnt stick to dimensional accuracy but to accuracy of fit. Hope you understand what I mean ;-)
Best regards
Guido
