No my intention to be curt or insulting.I apologize. As others have suggested, there is no clear cut formula for such a thing.
Personally however, I don't understand the importance that some place on gun valuations. It's a commodity to be used and enjoyed(and if enjoyment is derived from keeping it unfired,that's fine too) Whatever that value is when its time to go,whatever my luck is in finding a buyer, that's what it is.The same applies to countless other items.I'm not going to worry about now,and my estate will just drop off the stuff at the LGS,and we know how that can go at times.I also don't apply the word "investment", or +-%, to items in the price range of most post war Smiths.It can get to be a discussion over the value of a dinner,some gas for the car,etc.
Back to your example, I figure that I had the fun of shooting my 19-3 for several years,and the hit that I took for doing so,over keeping it unfired, was in the neighborhood of $50...maybe 7-8%. Not worth thinking about,and both fortunately and unfortunately, I have bigger things on my plate.To me,condition trumps fired vs unfired. I'll take a pristine fired gun any day over a NIB that has picked some marks.
I also don't see rigid values to any gun. You could take 10 identical NIB guns,(or 10 identical fired)place them one at a time on a sales site and not realize the same price for any of them. The fluctuation could be just as great as the fired/unfired thing, with perhaps a fired gun even fetching more than an unfired example..