For FA, I can also only charge 1 to fill, but more for other competitions. We don't know the formula for fan base, but can be your win loss record, or opponent quality... I think I've seen mine dip if I play a team with few less overall stars, but I haven't tracked it at all.

I'm confused too like @shire, I've seen 5% given when stadium is much less than full. In fact I first noticed in one of @Al's matches... Many empty seats but the bonus was full. Can't explain it