A little late to jump in this but I think I see the problem. It appears to be misleading information and a poor choice of words snowballing into a pile of confusion. I went ahead and searched for the other topic you mentioned which appears to be this (harem-battle. club/kamihime-project/3352-explanation-damage-calculated.html), where the opening post makes the same mistake you did. I feel like the others didn't really do a good job at clarifying anything either.
To make it easy to understand, let's simply do the calculations using your numbers.
Your two values are 2.4 and 2.8. You said that "you lose 40% damage" which is the sole problem. 40% of 2.8 (.4 x 2.8) is 1.12, so saying that "you lose 40%" is basically saying there's 1.12 difference between the two values, which as you can see is not the case.
The percent value difference of that .4 is 14.2857%, rounded to 14% by others and found using the formula mentioned in the previous page. So 0.142857 x 2.8 gets you .3999 (rounds up to .4) and matches the difference between the two values, so it's the correct percentage. So you would lose about 14% damage would be correct.
To use that .4 as a conclusive, you'd need to keep it as that decimal value and not a percentage as that will lead others to think it's a percentage of another value. So something along the lines of "You'd lose .4 to your multiplier" and not 40% to your multiplier or 40% to your damage.