It's but a rule of thumb. In detail, it's well dependent on the equipment.
As I said, you'd have to add a club or two instead of a few yards with Starters, e.g. at CCC#2 or OAK#2, not to mention BPB##2, 15, 18.
For uphill shots with my good G20s and Tour-SD balls I used no addition for shots without spin (but never 1 yard shorter). The ball would land on the green shorter than usual but with a smaller angle and just roll uphill to nominal length.
Now, the lvl 33 Callies need care with said formula to prevent from short shots. Their better spin would stop them shorter and influences the length more.
Other clubs & balls will need other care, I'm sure.