Sounds like a fatal hit to me. Where the "P" meets the trail again, I'd be willing to bet he may not want to climb very many more hills after traveling that far and losing that much blood. If you're going to do a grid search, I'd start where you lost last blood and downhill towards water from there. Not saying to rule out looking up over the hill again, but I'd be willing to bet he's traveling his easier path by now.
And when you think you have looked far enuff go a little further!
Aint that the truth!! helped track a buck the other day for my buddy. We looked for about 8 hours the morning after he hit it. We lost good blood after about 80 yards and then did a giant grid search. We must have covered every leaf in the county. We just about called it off and started heading back. We walked about 20 yards in the woods and I said Wait a minute I gotta check something out. We were already a long ways from where we had last blood, but there was a standing corn field about another 500- 600 yards across this big wide open field. We walked down the hill to it and spent another 2 hours walking down each row. The 3rd from the last row laid a dead buck!! He was hit in one lung and that's the farthest I've ever seen a deer go with a lung hit and die.
Good luck keep us posted.