I'd lay out the posts 8' OC. If you hit a big rock, pin it. The frost won't effect a rock that large. Where you can dig down 4', do so, the frost won't lift the post out of the ground in a few yrs.Yeaaaaaaaa they are like the iceberg that sunk the titanic. It looks like large but overcomeable size rock for the backhoe on my tractor but I would end up excavating a 6ft perimeter and pulling basically a car out of the ground. Makes for good paintball shelter in the woods!
I think my concern is that while rock like that is common, and I really won't know until I start digging so sounds like I need to just get cracking and determine what's going to work best for each post. Uniformity is overrated anyway!!