If I have an f150 lightning with less than 75k miles but it rebuilt from salvage what is it worth?

Less than it's blue book value, which you can lookup online at any of the following used car valuation sites: edmunds.com, kelleybluebook.com, nadaguides.com or hemmings.com. The salvage title lowers it's value even if the truck was 100% rebuilt, if only because it affects insurance.

Do you know whats the percentage it depreciates? I heard from some that it goes down by 20%

I don't think there is a set percentage like that. I've seen numbers as high as 50% depreciation for a salvage title. The biggest hurdles for a buyer of a salvage titled vehicle is: 1) insurance and 2) getting a loan. It can prove nearly impossible for a buyer to get financing on a high dollar used vehicle with salvage title. Your case might be different since yours is older. The insurance is another major factor. I rebought my own vehicle after is was stolen, and the insurance totaled it for the write off. I bought it from them as a salvage vehicle (it had no damage other than cosmetic). Then I saw my insurance was costing my nearly 50% more. Couldn't unload that vehicle fast enough, lol. Also saw first hand how much it depreciated due to that title status. If you're not in a hurry to sell, try listing for a certain price your comfortable with and see if you get any bites, you can always lower the price until someone bites.

Alright, thank you very much! The truck it's self is in great condition but the previous owner had a minor accident and they salvaged it because since it's a sport truck they didn't want to keep covering it.