Single Missing Tooth In Perforations. How Much To Discount?

This is a more general question about stamps in general. Forgive me if it shouldn't be posted here. If a single perforation tooth is missing from a stamp, how much do you usually downgrade its value pricewise from catalog value if you still want to purchase it to fill a hole that you've been trying to fill for quite some time? Just looking for a rule of thumb. I know it's really a matter of how much a person wants an imperfect stamp instead of no stamp at all.
Thanks

I would value the stamp at no more than 5-10% of catalog. A lot depends on how visible the missing perf is. Is it a full perf or just a short perf? Is it along the center of the stamp or in the corner? Mint or used? On cover or off?
And I would be more lenient the harder it is to find the stamp.