Secman, I am very sorry to hear about your seperation - it must be an extremely difficult time for all the family.
I don't have any specific advice re maintainance but have you cheked out the Single Parents Board on rollercoaster.ie? There are posters there who give excellent advice not only on calculating maintenance but more widely on how to mediate an amicable settlement on these things and how to best work through a co-parenting relationship with an ex when there are hurt feelings on all sides. You'll also get a very good steer on exact entitlements for single parents etc, so will be aware of what other financial supports, outside your own contributions, that your wife will have available to her. Discussions on Rollercoaster can sometimes get a little prickly but there are normally enough factual posters to keep things on the straight and narrow.
I hope that things go ok for you and for your children.