Depends on juice, build, wire type, coil type, etc. On prebuilt systems... I follow the recommendation. If I build it myself, I keep the "3.7V Sweet Spot" in mind. To find how many watts gets you 3.7V on a VW device, use steam engine.
Why 3.7V? No matter what, unless you're using a parallel box, and super sub-ohm (.09 and lower), no need to go above the "nominal" voltage capable of being delivered by the battery(s) base specifications.
Gunk on coils, caused by sweeteners in juice. Change juice. Try using TC or switching to 316L stainless (would say other stainless, but I haven't tested any other forms of stainless steel yet) wire. Since I've moved to 316L wire, gunk has been almost non-existent. Most of the time it's a juice thing though.
.4 ohms is pretty easy to get. Depends on gauge (wire diameter) you're looking to use. 20 gauge and lower... Don't even waste your time. Too many wraps or coil diameter would need to be wrapped around a garbage can to get desired effect of .4 ohms. Try 28 gauge or 26 gauge. About 7/8 or 8/9 on approximately a 1/8" drill bit would suffice. Could go larger inner diameter if needed to decrease wraps or smaller and increase wraps. Totally up to you.
If you're not comfortable wrapping your own coils on a rebuildable atomizer...