Makarov doesn't know anything. He is purely guessing. He didn't even know how strong Gray and Lucy have grown until they both powered up. Makarov is way out of his league at this point, after the timeskip, the age has really start to wear him down.
Sorry I wrote Makarov instead of Gray, so I'll re-state what I meant to say.
Gray said that Ajeel was the same level as the woman on Caracol (Brandish) - this is crunchyroll translation.
That is the most accurate piece of info we have on Brandish as far as I am concerned, because Brandish has done very little so far. At least on pure magic presence Gray has given us the most reliable bit of info by saying they are on the same level. Whether that is slightly under/equal/slightly above is impossible to tell.
If it had been Ajeel in that tub, even without Celestial Spirit Magic, Lucy could have ran out of the house and escape. It is the nature of Brandish 's magic that made it that she couldn't. I don't care about coolness, you cant just ride off the fact that Brandish's magic is stronger than Ajeel's. The magnitude of the magic also accounts for the ranking which is why it is obvious.
Brandish threatened Lucy to shrink the town, Ajeel would have threatened to do something with sand, August would have threatened something else etc etc. Maybe Lucy would have tried to run against August/Ajeel who knows. These are hardly reasons to say Brandish > Ajeel.
A noob could use Brandish's magic and compete on the likes of Erza or Gajeel.
How can you know that when Brandish has Spriggan level MP (if there is such a thing). A noob might be able to shrink a house and then drop to the floor after due to fatigue. Or do you think the noob can easily shrink an island?
In this series Dragon Slayer magic is the best or one of the best magic there is (better than Brandish' magic). Mastering this magic, and becoming more powerful (MP wise) is what will eventually allow a DS to slay dragons. But as we seen in the Grand Magic Games 'noob' slayers couldn't do much if anything to dragons. To me this illustrates the difference between a 'noob' magic user and a master (a master DS would slay or at the very least do considerable damage to a dragon).
We both know that Lucy had no choice but to fight, cuz she couldn't run away. If it weren't for Ajeel's sand, both Lucy and Cana would have been screwed
I do think Brandish would've wiped the floor with Lucy and Cana. But Cana wasn't intimidated at all by Brandish' magical presence - so should I use that to downplay Brandish? No. When you have Cana's reaction, and Gray's reaction - and the difference between those, it demonstrates how character reactions are not good indicators for power. These reactions are more for plot so that Mashima can make us informed/fear the opponent.
The 10% was an estimated data. It's obvious who has taken more stress at this point. Ajeel has shown to be huffing and puffing up till now while Brandish looks like she is watching TV.
Brandish has not even battled, or had to do anything. 10% though... is Brandish 10x stronger than Ajeel?
Of course all of them need to show more, but based on what we've seen, it is enough to justify our ranking.
It's fine to have Brandish > Ajeel as speculation, but we haven't seen Brandish outdo Ajeel yet. It's not her fault, Mashima is yet to use her. I personally like Brandish a lot but I don't hype her unnecessarily when she hasn't done much. She gets as much hype as DiMaria and Invel right now tbh.