In all honesty, I find rule 6 for damaging a Ju-88 (Table M-2) difficult to remember. One might achieve the same flavor (Ju-88's are hard to destroy) like this:
Remember that these modifiers are additive. For example, an ace gunner in the mid-upper turret and a non-ace gunner in the tail turret would both be at -1 on Table M-2.