Fellow Zenith builder here--
That used to puzzle me too, until I discovered that 'dl' refers to 'developed length', which as the other posters note, is roughly equivalent to 'bend allowance', ie, the dimension the part needs to be made to on a flat sheet of aluminum such that, when bent, it has the desired finish dimensions. From what I've gathered over the years following the Matronics lists, the Zenith 'dl' dimensions work great for hand forming over wooden forms, but less so if you're fortunate enough to be using CNC machinery to cut and bend the parts. In the latter case the bend allowance has to be determined vis-a-vis the equipment you're using, and/or by experiment.
What I'm curious about is whether this is a commonly used term in aircraft building, or a more common overseas term? Here in the U.S. I've always heard this referred to as 'bend allowance'. It wasn't until I started building the Zenith (in metric) that I came across 'dl'. I've found reference to it in French-Canadian and European sources, such as this online dl calculator:
so I'm not surprised Chris Heinz used it, being a Swiss native, Airbus veteran, and having worked in Canada. But I've never seen it used in sheet metal shops here in the states, and I'm curious if this is a commonly used term by aircraft manufacturers in the U.S.? Anyone know? (And sorry for the thread drift.)