Our recommended maximum on a K40 is 15-18mA unless the tube has markings indicating a higher max current. 15mA for longer life; 18mA if you are willing to sacrifice a little tube lifetime for more power, I think.
However, you don’t necessary lock the potentiometer at that rating.
For example, if you are planning to raster an image, you might set the potentiometer lower so that it’s the darkest you want on the particular material you are using for that particular job. That gives you maximum contrast range and smoothest gradation of contrast on your raster engrave. You just don’t want to adjust it above that 15-18mA range no matter what. Similarly, if you are vector cutting, you are likely to want to set it to a constant power setting appropriate for cutting; if you are cutting 2mm acrylic sheet you might want a lower power setting on the potentiometer.
Does that help?