I just figured out how the HP increases based on the constitution stat. It goes in a 10-20-20 cycle, so when the constitution stat is 6, 7, 8, your HP becomes 110, 130, 150. When your constitution stat goes to 9, it returns to 10 points, so for 9, 10, 11, HP becomes 160, 180, 200.
Tldr, you get 50 HP increase for every 3 points in constitution with a 10-20-20 cycle for each point.
Now to find out the maximum number of companions you can bring with you based on your charisma stat. Also would be nice to know how keensense is used in ranged weapon damage calculation.