And actually, my insurance company (in Massachusetts, I admit, so rules may be different) gave me /the exact estimated amount/ that they'd, well, estimated the damages at, so I would look warily at their being willing to let you keep the car, but for a smaller amount.

Also, at least in MA, even if the estimate of the damages is within 75% of the total value of the car, the insurance company is allowed/required (not sure which) to total it, so even if you get a small reduction in the estimate on a second opinion, it may not help.