I just ordered another kilo and found many places out of stock. Apparently word is getting out it's good stuff.
That "effectiveness" term will be echoing confusion through these forums for years to come.
YAN is a defined term - yeast assimilable nitrogen. It is measurable. Manufacturers may provide the information. In the case of Fermaid O, 1g contains 50mg of YAN. If 1g of Fermaid O is added to 1 liter of must, it will add 50 ppm YAN - end of story.
I think it is good if we keep our facts and measurements straight.
If a person believes that the effectiveness of amino/organic nitrogen is greater, such that you only need 25 ppm to have optimal fermentation, they are entitled to their opinion, but I haven't seen much scholarly data to support that assertion.
Lallemand state that 30g/hl provides 30ppm here http://www.icv.fr/en/oenological-products/activators-nutrients/fermaid-o therefore:
10g/hl = 10ppm
0.1g/l = 10ppm
1g/l = 100ppm
This is a suspiciously round number and is also the exact number quoted for fermaid k at least in zpeckler's link, which makes me think Lallemand are not 100% accurate when quoting yan ppm in their products...
I still do not know how much yan yeast need and how much yan products provide. From what l've read the TOSNA approach seems the best way to go in most cases. I might re-evaluate for high requirement yeasts or for high gravity musts