I think there may be a way to do this at home, but it would require some specialized equipment at the value of a few hundred dollars or more, and the specific know-how to use those pieces of equipment. It's more of a winery-scale sort of thing still. I'm waiting for a way to measure YAN at home, with baited breath.
I don't make many kits, honestly. I think I've only ever made one.
How much I add depends on what I'm working with, and it takes a little research coupled with a few years of experience.
Every batch will have different YAN levels, so I start by looking for published (read: not a blog or personal take, but scientific white papers) evidence of the amount of YAN in the produce that I'm working with and couple this research with a physical take of the fruit. Is it optimally ripe? Or was it picked early? Is there a lot of visual disease or pest pressure indicators on the produce. From there, I can feel comfortable roughly scaling the fruit using the research as '100%' and scaling back based on any deficiencies I've found.
The second part of this is looking at the yeast I'm using and where it falls on the scale of low-medium-high in the chart, in that Yeast Nutrients post. That scale somewhat directly relates to necessary YAN levels needed for those yeast to ferment ~25 brix without issue. I say somewhat because some of this research is still being done or hasn't been published for the general public. But if the yeast requires more nutrients for optimal performance than what I've guesstimated the fruit to contain, I know I need to be a little more heavy handed with the Fermaid-O and not hold so strictly to the 3x efficiency rating.
Once I've determined that much, I look at the amount of sugar I'm trying to ferment and where that is in relation to the yeast nutrient requirements, as well as is it over or under the 25 Brix that the yeast nutrients as scaled to. That can push or pull on the overall amount of Fermaid-O I'll be using; if it's under 25 I can get away with less but if it's over 25 it's going to be more nutrient needed.
Knowing that most fermentations need at least 200 ppm YAN, but most don't need over 350 ppm, and basing these assumptions along the low-medium-high scale used for the yeast chart, I can ballpark the amount of YAN I'll need to supplement by subtracting the assumed YAN level of the fruit from where my guesstimate put me. This is general YAN though, so then I can divide that by my efficiency factor to adjust for organic/Fermaid-O supplied YAN.
So then I have an idea of how much YAN the fermentation will need, but this is in no way a 'hard number'. Because Fermaid-O is assimilable farther into fermentation than inorganic nitrogen, I split this total into 3 additions - one after the lag phase, and the other two at 1/3 sugar breaks. But, the catch is, that I watch the efficiency of that first addition. Did the ferment make it from the lag phase to the 1/3 sugar break, without any H2S, without appearing to stall, without any issues? If it did, I'm generally in the right ballpark for all my calculations. If it didn't, if there's H2S or if the cap sinks on me or doesn't reform after a vigorous stirring -then I know I'm off some and need to compensate on further nutrient additions so I don't ruin the batch with fermentation difficulties.
