As said, the lowest setting that gives you a good hit, every coil and oil combo is going to be a bit different. The trick is to keep air moving through the coil while it’s on, preheating or not, and to hit it slowly. Don’t try to breathe through it, it’s not a bong. Sip on it, take short hits with a few seconds between them. The oil is pretty thick, it’s very possible to starve the center of the ceramic heater of oil on extended or hard hits, that causes this crusty burned residue to build up and the coil stops hitting right.

Slow and steady, let the coil refill with oil between hits.