That formula is wrong. I had someone test at level 35 ranged and your formula would give floor(39.2+2) = 41, when in reality it boosted to 42. The evidence tends to support floor(1.1x + 4) but I'm not altogether certain about this!
What's the formula for calculating the boost of a ranging potion? A lot of sites say different things, so I was curious as to the final authority on this.
I started 07 with my only goal being getting 200m Agility. I halted that to try to become the first player to max instead. I was the first to 99 Agility which meant that I'd inevitably gain additional post-99 Agility Xp from Fishing and Runecrafting. I did about 800k past 99 at first and then obtained over time an additional 1.3m+ Xp from the above mentioned skills. You really, really like Agility!