Not much to go on to be honest. Naeff all we know for certain is that he was not as strong as Nynaeve (that's stated when Nynaeve and he are cleaning up the bubble of evil in Tear just before she Heals his insanity). So I put his maximum level as a 5
Narishma's minimum level is based on Merise's comments about his growing stronger, and her worry that he would become stronger than she was (level 14), implication being that he was at least equal to her currently. I placed his maximum potential at ++5 mostly to be tongue in cheek ... Sidious made a comment in an earlier post that he'd like to see people in the ++5 and ++6 so I threw some in there (False Dragon's and Narishma).
Taim I placed at ++3 because we know he was weaker than Rand, and it seems likely that he was not as strong as Demandred, who recruited him.
I have been adding rational into the placements, but it's ALL thin as I haven't had time to really go hunting for specific quotes beyond what's in the Companion at this point. So mostly it's based on knowing if someone could Travel or if a woman was strong enough to attain the Shawl or if she was directly compared to another woman whose strength we know. Or has a feat of the power directly comparable to another whose strength we know.
I don't think you should add assumptive strengths.... What I like most about your list is that it's factually accurate. In my opinion Taim is ++2 but like all the other Ash'aman except Logain we will never know. I'd like to use your list as a reference so maybe try keep it "perfect"
Keeping the factually accurate list as is and adding in assumptive levels as a separate piece