the concept of optimal peep differs in different lung conditions,more for atelectatic lungs,can be counter checked with x ray for lung spaces,though dr wung has described the optimal peep of atleast 5 for all his cpap babies, i guess we can regulate with the case scenario, but it might be prudent to avoid overinflated lung,reduced venous return, barotrauma at one end and underinflation of lungs at the other