I've got a professional base station that's decently close, but still far enough to degrade the accuracy a bit (I forget the exact figure but you can find it if you search). When I used the professional base station to figure out the point and compared it to what PPP had found, it seemed to be within that amount of error.

I chose the part of the roof least likely to be disrupted. But if I do need to move it, then I'll have to figure out exactly how I want to map to points going forward.