The YAN levels themselves, can be hard to track down. When it comes to raw fruits, sometimes you can find literature for that particular fruit that will actually give you some rough numbers to go off of - like I remember finding the numbers for YAN levels from highbush blueberries. So in that case, it would come down to the ripeness of the fruit at hand, to assume a percentage of the YAN levels listed in the scientific literature. For something like Welches, it's practically impossible. Either way, its still just a ballpark guessing-game. There is equipment to do the testing on YAN levels; I dont own any of that equipment to really speak on it, but I know they mostly test things in a juice form, so if one had the testing equipment, they could figure out the actual numbers to something like Welches.
That's kind of where that particular chart can come in handy. It gives you a base to start with. If your hydrometer measures out ~27 Brix, you can roughly assume that it's going to take ~350 ppm YAN to handle that amount of sugar. You can factor in the ingredients from there, and let your gut tell you whether or not you think you need to under or overshoot that 350 ppm YAN marker. The yeast choice also can play a role in that decision, but 1118 is pretty beastly once you get it going.
Since there's no real raw fruit included anywhere, I would personally overshoot that marker a little.
So you figure out, like you've done, the amount of YAN you need, and get that number into the number of grams for the product you're using. Then you've gotta decide how many steps you want to break your nutrient schedule into. There's always an addition after the lag phase, when the cap forms and fermentation really happens. But from there, it's sort of flexible. The yeast still need to eat, so you cant wait too long but if you want to only make 3 additions, you'd have one addition after the lag phase, another addition at say the half way point, and the last one when you chaptalized. These 3 additions would be larger in gram-size, as opposed to if you wanted to do 5 additions - lag phase, half way, and chaptalize three times. Same total grams, as you calculated, just split up into a different number of additions.
Go-Ferm is done per the instructions that it comes with, or if you're fermenting a larger batch there's an ratio (I believe its 1.25 grams Go-Ferm per gram of yeast, but dont quote me; its listed in the Yeast Nutrients sticky though) that you use. It's done during yeast rehydration and is separate from the rest of the nutrient additions.
Hope that helps a little bit
