I use two spokes also, but I don't cut off the j-bends. I use a caliper and squeeze the two J-bend ends together, do the proper math. This way I'm not interpreting what a ruler says and wondering if the two spoke ends are perfectly straight or not. Using the j-bends in tension I get a consistent hard number from the caliper.(as consistent as one can hold it that is !) The benefit of doing yourself is in you can use the exact spokes and nipples you're building with, and the proper threading end point of the spoke in the nipple. Do this in a couple of places on the rim for good measure.

The six mm thing sounds like "another one of those" wonky myths/misunderstandings that go around not in cycling but in Life in general.