What a strange way of cooking. I'm not saying it's bad, just that typically people cook for a result (I want to make X servings) not a reason (I want to eliminate all of the butter in my house).
Of course, this only works if you don't mind having too much food. I like leftovers (I get to eat without having to cook!) so it works for me.
I just round & then convert in my head though. (Eg 30/220 ~ 15% = a tenth + half a tenth).
I don't cook things without purpose/specific goals. I'm cooking to make enough for the people I'm expecting.
That's not to say it doesn't make sense, just not how I've tended to see things cooked.
- If it's something that can be reasonably divided among guests (such as a soup or something similar), then I eyeball the portions so they're all approximately even. Optionally, if there's an ample amount, you can simply let people decide how much they want through self-service.
- If it's something that's demarcated by physical objects (e.g. dinner rolls, cupcakes, etc), then you have a few other options: Leftovers to save for later, split them among people who want extra, or give them away to whomever wants them.
I don't know about the OP you're replying to, but I rarely divide a recipe up based on expected portions since portion size is highly variable. Should I have extra left over, then I deal with that accordingly. If I'm cooking to get rid of an ingredient, I don't particularly care if I miss the mark by a few portions provided I have enough in the first place for the objective.
(The other problem is that portioning in this question seems to me to assume that all guests are equally hungry.)
Let's do this as a math problem.
How I tend to cook: Xbutter + Ysugar + ZFlour = 20 Cookies.
How OP is cooking: 2 butter + Ysugar +ZFlour = X
We're setting different known quantities (him butter, me portions) and then solving for the rest. They're fundamentally different approaches in cooking philosophy. And again, neither is "wrong" I just found OP's style to be novel enough to comment on
Basically, the reason is, I want to make as much as possible of something from the ingredients I have. Just find the constraining ingredient, calculate the ratio, and you're set.
Again, clearly this is some kind of a thing some people do, I just don't tend to cook this way apparently.
Throwing a spanner into the works, just for fun:
In India (and probably some other countries), cooks often don't measure out ingredients for dishes [1]. They just wing it / use their experience [2] (called andaaz in Hindi/Urdu). And the results are often or mostly good (except with bad cooks, of course).
[1] I guess that approach works for all countries, except individual cases where people blindly follow formulas to the letter.
[2] Of course, they are not flying blind, they use rough (or precise enough) estimates, that are based, again, on their experience, and on just seeing what amounts of what stuff (whether it is pinches, grams, ounces, kilos, pounds, or whatever), makes for a good dish.
That's a whole different style of cooking, and most people don't get there until well AFTER they learned how to cook something with a recipe (or someone telling them how many handfuls of beans to add to the pot).
I guess it also makes more sense for baking than cooking.
All ingredients move together in ratios (got that, totally understand) so I understand HOW this work. What I was commenting on was WHY someone would cook this way. Again, I'm not saying it's wrong, it's just a problem I've never encountered. If I only used 150G of butter and had 100G left, then I have 100G of butter for whatever else I'm cooking.
I'm sure scenarios can be presented for why this is a problem that needs solving (storage space, product expiring, etc) it's never been a way I personally did things.