On page 274 (DMG), the following is found for defensive CR (with an equivalent for offensive later on):
If your monster's AC is at least two points higher or lower than [its hit points suggest the AC should be], adjust the challenge rating suggested by its hit points up or down by 1 for every 2 points of difference.
What does it mean by 'points' here?
Take an example where a creature's AC is 16, but it has 36 HP. It's 'hit point CR' is 1/4, but its 'AC CR' is 8.
Do you count \$9\$ rows up on the table to get \$-4\$ (and therefore CR 4), or calculate \$ 8-\frac 1 4 = 7 \frac 3 4\$ to get \$-3\$ (and therefore CR 3)?