I see many websites do the conversion as 1/2 lb -> 226,79 grams but guess what, I am not going to the butcher and ask for 226,79 grams of meat. Just like I won't do 14,79 ml of oil (1 tbsp) (well, I just use a teaspoon in this case but you get the idea).
Labels are clearly converted from F but with some non-perfect formula, 700F is actually 371C not 374C for example. Still trying to figure out the formula used.
the difference between 38℃ and 94℃ is 100.8Δ℉ rather than the 100Δ℉ that was probably there in the original markings. It's the closest you can come to the correct values though, 93℃ is off by 1Δ℉
They then assumed 100Δ℉ = 56Δ℃, and applied that to the rest of the dial.
That means the values on the dial are off a bit, and should be: 38, 93, 149, 204, 260, 316, 371, 427
Seriously, virtually any ingredient can be off by as much as 5-10% and often more, it won't matter THAT much and if it does, you can adapt.
"Add one small onion"
Oookay...how big is that? How much of a difference in flavor will it be if I add more or less? What if I barely like onion? If I add more, will it cook enough to not change the outcome significantly?
It does take experience to guess whether it will matter or not, so if you're getting started it's nice to be given a clue (like weights).
Also, adaptation assumes that you're going to be trying the recipe again to get it right for the next time. Some people may do that, but there are so many great recipes out there that a lot of people (myself among them) will just go find some other recipe if one doesn't turn out well.
Go to the store and look at the onions. Categorize them as large/medium/small. Now you know what a small onion looks like.
Additional precision doesn't help here because onions aren't all the same. Two onions of identical size will contribute varying amounts of sweetness and pungency based on their freshness. The effect of your knifework and cooking temperature will make the outcome vary as well. "Add 200g of diced onion" is false precision. It looks more precise but it actually doesn't change the reproducibility of the dish at all.
> How much of a difference in flavor will it be if I add more or less?
Is onion a main ingredient like in onion soup? Probably a meaningful amount. Is onion just part of the overall mix? Probably not much of a difference. This isn't something that a recipe can tell you. This is something that learning to cook tells you. There is an entire genre of books that teach this "what makes recipes taste the way they do" concept but they aren't recipe books.
Yes, but not an intractable amount of experience. There are very few ingredients for which inaccuracy of measurement yields a clear "pass or fail" to the dish. In most cases, you would just make a mental note to add more or less of whatever ingredient it was next time.
Accuracy-sensitive ingredients that come to mind are usually powerful spices like asafoetida, saffron, cloves, garlic. Or, in baking, leavening agents (which can usually be compensated for with time and temperature). There's not a lot of them.