I think he is referring to the inside the tank "straw" height for the Main/Reserve feed. There seems to be a rubber or plastic sleeve looking thing at the bottom of that straw, and if that is leaking, it won't matter what height you set the straw at, cuz it will drain the tank dry anyways. I have noticed on a petcock I am working on (and assume most, if not all are the same), is that, that straw is off center, and SEEMS that either from age or ethanol (or both), that the "ring" that is SUPPOSED to seal the main FROM the reserve section won't slide over(and seal) where the straw goes into the body of the petcock.
...And I'll betcha a dollar to a donut that many who are expieriancing a "NO RESERVE" from their gas tanks, that that sleeve has hardened, or cracked or swollen, and will no longer seal.