I was curious and looked into this a little.
From what I see, insurance companies have a formula they use to determine diminished value. Now attorneys will tell you the formula is horrible and should never be used. I'm sure insurance companies will say it is beyond fair. Probably somewhere in the middle.
The real point is, the way the formula is structured, you are likely to see little on a diminished value claim. Also, since it has been accepted by various courts, you will have a very hard time overcoming it. I say this not to say you shouldn't pursue it, but that you should expect it to be difficult and you may have to decide if the effort is worth the potential reward.
How insurance companies calculate diminished value claims - Employment Law | Family Law | Bedford | Fort Worth | Dallas
FYI, this is written by an attorney who goes after insurance lawsuits, so it is definitely biased as he sees things. I am willing to accept he has accurately represented the formula, however his commentary is definitely anti-insurance company.
Here is another article on a mechanics' site. Again, not exactly pro-insurance company.