To some extent it is a negotiation and not a dictatorship, but you've got to be reasonable. If you have legitimate evidence that your particular car is actually worth more than the assessed value, the adjuster should be willing to review with you. However something goofy like "hey man, I've got this sick exhaust system, that's worth $2g, and I just filled it with gas"...lol...well that isn't going to get you anywhere.
In your example, I'm assuming Y is simply X minus your deductible?
What kind of guidance has your broker given you?
"Masked Bandit is a gateway drug for frugal spending." - Unknown303