75 to 70 seems ok. Otherwise he will be very close to Zodiacs.
And you have some points wrong. He didnt say he used 35% of his power, he said he used 65% and was at 35 of his Nen. Kunckle didnt say his aura was the highest, he just used it as reference.
Feitan also should be in the 70-75% range. As Hisoka doesn't really see then as a challenge, he only was surprised by 95%.
Also, this is by how much aura imo they external, it also may not be a real indicator of how strong they are.
And by comments, they really arent the fighting type, Feitan IMO performed better than Morel, but still are near each other, he should still be below the Zodiacs which the weakest is 77.
I dont think the strongest and greatest hunters will just be equal to a mere criminal group. So Feitan and the most are below them just as Morel.