Sadly, that means wasted developer if you use Jobo tanks. I've just bought my first bottle of DD-X and have been pondering the same question. I can either develop one roll in a two-roll tank or three rolls in a four-roll tank (35 mm). I believe in filling the tank for inversion agitation, otherwise the sloshing introduces a new variable. So my choice is like that of the person who posed the question - use less DD-X than Ilford recommends, or use the recommended amount and not get the aforesaid 16 rolls per liter.