The multiplier is where subjectivity MUST occur. There simply is no way around it.
Thats what committees are for... Ideally the group pools resources to reach a valid conclusion. Unideally it gets stuck in the mud and accomplishes nothing.

The ITA CRX is an example of a car that was underrated from the factory. By the time the "realignment" came around we knew this pretty universally, and the multiplier was adjusted accordingly.
We won't always have that benefit, so the opportunity to "fix" mistakes is a must. It just is.

What we also can't do is then take what we know about (for example) the underrated ITA CRX and arbitrarily just start applying it to all Hondas. That invalidates the whole procedure and falls back into the whole classing via voodoo trap.
You know what you KNOW, and thats all you KNOW.

The reasonable target is 25%, but there should be no issues with using 15% or 35% or even 40+% (see the aforementioned factory underrated situation) IF THERE IS REASONABLE EVIDENCE THAT INDICATES THATS WHAT SHOULD BE DONE.

Again, subjectivity is unavoidable. But the goal should be to be able to use actual numbers to justify why a specific car is speced the way it is speced.

Another way to put this is if you have speced a car at a weight that will likely make it have to carry 100+ pounds of ballast, you need to be able to say why it has to carry that ballast.
Currently that can be done for a few cars and thats it. Most can't be justified, and many of the ITAC guys will honestly look at you and shrug because even they can't explain why.