Let's just assume you're right about the 90-95% not helping, at all...that won't be a problem because I do not charge him anything (+ I have to get started somewhere).
I'm often called an overanalyzer and overthinker for doing these kind of things :-). I do have certain goals in mind, but as I do tend to overanalyze/overthink stuff (Im not one of the folks who act before they think, but who thinks (too much) before he acts!), I dont think its a bad idea to just get started to get the feel for this, and how it affects his results. - Keep in mind how careful I am (asking all those SE guidelines questions, trying to make sure his site would still pass a handcheck,etc.).
I know his rankings, and the first goal I have in mind is coming up with one successful link building campaign (after having improved his on-page optimization to rank for some uncompetitive+highly targeted phrases he hadnt considered already). And then I'll see the effects it'll have had on his site's rankings for different search terms.
If, they havent improved at all or not enough, it'll be time to re-consider what I do. Track, adjust... I dont plan on spending 12 months on link building and then wondering if it has worked and coming on here crying about how nothing works ;-).
I hardly doubt that a successful link building campaign the way Im trying to do it wont help his rankings if I get it to fly, considering the links he has built have improved his rankings and gotten him top10 rankings for his city/state + business niche (mostly reciprocal and directory kind of links). ..Of course, I cant exclude the possibility that the l inks he built over time have nothing to do with it and that it all comes down to his site having aged....in theory.........
Ill spend 2 weeks on this and track his rankings, if they dont improve I know it wasnt worth the effort and Ill have to adjust.
PS: As for not giving away much/enough about the website at hand - it might seem a bit childish or something - but he did ask me not to give his niche away, as he's looking to scale it,etc. ...whether thatll work out ofr him or not is another question...but if he trusts me that I wont name it, I'll have to do just that..