Don’t target a body fat percentage. Train lots, eat well and then see what your BF% is and notice how you are performing. Going too lean will have a negative effect, maybe even more than carrying too much fat. Based on my experience; top male XC skiers will fall in the range of 4-7% typically. Going much below 4-5% often results in poor recovery, more illness and a drop in performance. But find the BF composition that allows you to handle the high training load without breaking down.