Uniform wear and tear on a firearm can be estimated and evaluated, but a "bad spot" detracts significantly from the marketability of a gun. Touch-up blue can mask such a spot but is considered borderline unethical in most places.
You might try to sell it as an "honest gun" at about 80% of the new-in-box price price and see if you get any takers. A lot depends on how extensive the other dents and scratches are. I hope this helps.

Age ,patina,engravings,grade of wood. You stated that it as a "Major" bluing loss spot, meaning that it is not from wear,but either defect in bluing or abuse. overall wear due to loss is expected as stated above.And as Treestand asked. These all effect value. But the 80% of box is far off. With bluing issues,dents and scratches you have to figure in repairs,which lessen the value as being restored by at least 15% if not more. Out the door loss is another 15% loss from the box cost. Now figure in the "Cost" and not the retail price which is probably,depentant on the firearm 25 to 40% below retail. $1000 gun drops to $850 walking out the door,$700 after usage, $600 after defects and so on depending on how bad. That is trading it in. If the gun is something special,untouched,old as in older then 25 years or antique with patina and not refinished the value increases. If it is a low production,special edition,special grade or attached to somebody important it goes up considerably. A gun that cost $100 a 100 yaers ago can be worth $100 or $100,000 all depentant on what it is ,what it has and history involved. Complicated huh? Good luck and let us know a little more about the firearm you are trying to get a value on.