1) John is 65 years old and needs the tree in his yard cut down before it destroys the foundation of his houe. It would cost him $10,000 to have a pro do it, so he decides to do it himself. He cuts the trunk on the wrong side and it fall on a car driving by, causing severe injury to the driver.
Q: John's duty was to observe the standard of care of...?
2) Bob has taken a class on emergency medical procedures but he has no other medical training. His neighbor collapses on his porch. Bob runs over and decides to perform an emergency surgery with his portable medical kit to open up neighbor's airway. Of course, this goes horribly wrong and neighbor croaks.
Q: Bob's duty was to observe the standard of care of...?
...reasonable precaution to prevent foreseeable harms.
...reasonableness varies with the inherent danger of the activity.
Actors with a higher degree of knowledge have a higher degree of reasonableness,
so in #1 I would think that a reasonable person would realize they need to account for which way the tree's going to fall. It was reasonably foreseeable that the tree may fall into the street. How foreseeable that the tree actually hit a car is not relevant, just that the tree might fall. I guess that B<PL could also be applied for breach saying that B=10000 and P would probably be at least 20%. L could vary quite a bit...
#2 I would say that Bob had a higher standard of care than a reasonable person. He had taken the course, which increased his level of knowledge and with that his standard of care, which would lead the reasonable person who has only taken such a course to not attempt surgeries.