About an eighth or less above the wick tube is the usual exposure, but you can't really see that dimension when its burning. When you first light the lantern, adjust the flame so there is no penumbra showing above the cone. The penumbra is the dark area below the bright part of the flame. A tubular lantern flame will increase in intensity as the lantern warms-up. Check the flame after 15 minutes and you should see some penumbra above the cone. At this time you may want to readjust the flame. If you keep the penumbra height to 1/4" or less, you will not get smoke or soot.