The thread linked here is incredibly useful. This isn't a formula as much as a way of thinking about how to charge appropriate freelancing rates. Do not sell yourself short! https://twitter.com/MsMinotaur/status/1259628201199820801

Don't forget to add a small "the client is from a a far richer country" bonus too if applicable too. It's easy to calculate fees purely from your economy, but if your client is from a far richer place, adjust your prices up a bit. They won't notice, and it makes a difference.