Overall I can be attracted to most sizes, from thin to really huge, but what I consider as the ideal, it seems to slightly fluctuate according to my mood. Most of the time it's where she is jut big enough to not be able to hide it no matter how she tries to suck it in, up to where things just start to not be designed for her size.... but sometimes I do feel more attracted to a size which is just below a size when living independently would start becoming a struggle.

That will happen in billions of years. But pollution can ruin the planet in a century or two. That's a significant difference.

I think it's not really about the raw numbers, as even compensating for height, people can have different body types, different fitness levels, muscle, etc.

I would define the border between BBW and SSBBW more in the terms of how her size affects her everyday life, and how it compares to sizes smaller or bigger.

The effects of her size on the body, what it can do and where it can or cannot fit, would be a better criteria.

I would consider the threshold between BBW and SSBBW to be where one starts getting bigger than most things are designed for. Chairs, cars, seats, booths becoming too small to fit into comfortably (or at all). Certain body positions and movements starts becoming very difficult or impossible (tying shoelaces, walking up more than a single flight of stairs without having to stop, etc.).

I would consider someone a SSBBW from a size where one has to start making significant adjustments in daily routines because things start not being designed for that size. Where going through narrow doorways, fitting inside small cars and behind restaurant booths is still possible, but only barely and with great difficulties. Where every physical activity is still possible, even if only for very short periods of time. A size which starts seriously affecting what one can or cannot do, but still not as big to make most activities impossible.

Here is the rough idea, with the help of pictures: fantasyfeeder.com/forum/posts

It's no longer available.
Couldn't it be posted directly, in a simple text format?

My fantasy: "converting" a big but shy woman to feederism. No tricks, no cheating, no unethical methods, just simply showing love, care, and adoration. She then turns from someone who hates herself for heir weight, to someone who accepts her weight, then slowly to someone who enjoys her weight. At the same time, she goes from trying (but failing) to control her huge appetite, to a phase of just letting herself go, and then graduating to a phase of deliberately overstuffing. Her goals would change from wanting to get thin at the beginning, then abandoning it and just wanting to drop a little bit of weight, then just wanting to maintain, then just wanting to live her life to the fullest not caring whether she keeps gaining weight, then finally to actually wanting to get even bigger.

Stay away from these malicious links, and help report the user. The more reports, the sooner he will be banned.

Isn't this counterproductive? The purpose of sweat is to evaporate and help you cool down. If you wipe it off, you will overheat faster. (of course there are situations when it's more useful to not look too wet, but still, on average you overheat faster if you are constantly wiping off sweat)

Does anyone else experience a regular fluctuation of preferences (as a FA) or goals (as a feedee)?

Weight is not the single most important factor for me. I can get attracted to girls who are stick thin and also to those who are just barely mobile (complete immobility is a turnoff).

But the "ideal" weight fluctuates slowly but regularly, between just barely big enough so that she can't hide she's overweight with clothing, posture or sucking the belly in, and the biggest weight she can still take care of herself with even if only with great difficulties.

You never even tried testing the waters to at least know if it's a sensitive topic for her or not?
Just a casual comment in the style that women don't all have to be skinny to look beautiful, or something similar, might provide an opportunity to gauge her stance on the topic, without being too intrusive.