Ralph I believe you read that out of context.

"Basically it controls fuel down to the point where airflow through the carburetor is too low to effectively drag gas out of the Main fuel feed Venturi. This is usually below 1/8 total throttle blade opening."

This means that the Main controls fuel flow down to about 1/8 throttle blade opening, below which there is not enough air for the Main to properly work. Above that point it works very well usually. Sorry if I did not properly correlate the two sentences.

Setting the main needle, which is just starting to lose it's effect, so that it no longer dumps fuel properly at this point will give you an accurate indication of just how sensitive the Main jetting is to air flow variatinos. If you are too rich between 1/8-3/8 throttle, it's usually because your top end is too rich (In my experience) and when starting at half throttle, and backing the throttle down, you will run into evidence of a too rich top end mixture at the point where crossover begins to occur, usually in the 2000-2200 (or up to about 2500 sometimes) rpm range. The theory and practice here shows that your main jetting will still be supplying fuel, but the fuel will not be atomizing properly due to lack of airflow, and this will lead to overly sized fuel drops hanging in suspension, puddling in the crankcase, and not burning properly, leading to burbling and an overly rich indication. So, this is a good indication of just how properlyjetted your main jetting is. Just something I've noticed over time, and which has proven to be a very accurate indicator for me.