The best way to determine value is to go to Gunbroker.com and see what they have sold for over the last 90 days. Compare the pictures of what has sold to your item to determine if your value should be higher or lower.
The Blue Book of Gun Values was always a rough guide, but the current market has made it badly out of touch with current pricing. I retired from running a gun store in March of this year. I ran it for 11 years, and Gunbroker was the most reliable way to determine value.
If you are not a member, you will only be able to see asked pricing and current bids. However, you can still look and see what is currently being bid, watch an item, and see what it sold for.