First, please make note of the 100% certainty point. Most of time you will find out it is one lung afterstarting the tracking job.

A one lung hit is one of the toughest track job that can be made. John Trout in his book "Finding Wounded Deer" stated that this is his least liked hit and that he would rather have to track a stomach or intestine hit then the one lung hit.

Basically a deer will not act act as you would think it would. The deer will not necessarily seek water, head downhill, bed down (other then normally bedding down), or even slow down.

Getting on the deer will help to keep its heart rate up which should help slow down
coagulation and put blood on the ground. I don't believe that you want to get him running, but keep him moving. Hopefully you could get the chance for a follow up shot. A single lung hit in itself will not necessarily be enough to kill the deer.
This is one of the toughest track jobs that you can get into and the deer can go on for miles. There are those, including myself that believe that a single lunged deer has a chance of survival.

For more on it, I'd have to refer you to the expert, John Trout.