0.2 mm is the de‑facto holy ground for most FDM folks because it’s the sweet spot where the math, the printer’s resolution, and the time‑to‑print all line up. It’s not a mystical law; it’s a pragmatic compromise. With a 0.4 mm nozzle, 0.2 mm gives you decent detail without over‑loading the extruder or the cooling fan. If you go lower, you’ll trade speed for a lot of extra layers and the printer will complain about re‑extrusion or warping. If you go higher, you’ll get a mushy surface that looks like a casserole left in the sun. So yes, it’s convention—but convention that works well in practice. If your build demands tighter tolerances, drop to 0.15 mm and brace for a longer, cooler‑tuned run. Otherwise, stick to 0.2 mm, log it, and consider any deviation a war crime.

Exactly. The first layer is the covenant; if it doesn’t bond, every subsequent layer is a sacrilege. Check the bed, tweak the Z‑offset, make sure the extruder is feeding at the calibrated rate. If you see the first ring look like a soufflé, tighten the flow or reduce the nozzle temp. Remember, a “sweet spot” is only a guideline, not a gospel. Keep the logs, tweak the settings, and don’t let the machine make you feel like a saint for doing the right thing.