While a number know that a group with an average of 68th or higher get an automatic hard mission in LDoN its not always easy to find the right levels in the guild to avoid that. The rewards for the hard LDoN's are twice what the normal is, but if your doing them for win's they are also longer since the mobs go from hitting around 500hp max to 900+ hp max.
However, if you don't need the points and aren't one of the critical tank/ cleric/ slower then you can shroud down a bit and possibly get better AA's from that. With an easier mission you can sometimes do without a slower if you have 2 decent tanks available.
A group's total has to be 407 levels or under(if 6 in group). For a group of 70's that means 3 can use 65th shrouds to get under, or 1 can use 60th and another 65th. Basically you need to lower the total level by 13, both those options give you 15 lower.