I use a sliding scale. Got a like new model unmodified that is no longer the latest generation? 50% of list seems reasonable. Is it the latest generation NIB and not in stock at the retailers right now - maybe 80-100% of list.
Got a high end gun that involves long lead times or is NLA? 80% maybe more. Have targets or good competition results with it? Fresh from a reputable tuner? Those can add some value too.
If you’re accuracy focused like me, you’ll probably agree with the idea to never sell your most accurate rifle. So when I see nice guns for sale used I always measure the possibility that it’s moving along because it’s below average accuracy. I’m not saying that used guns are less accurate- not exactly - but I think it certainly isn’t helpful unless the seller is proving it to be an accurate copy.
Got a highly modified project gun that is kind of unique? Unless buyers are looking for that particular outcome I’d bet 50% of list and zero adder for the work that went into it.
I try to think like a buyer. What are the risks and guarantees here? If I spend more and buy new with warranty what would that look like? Then I price with that and my urgency to close a sale.