There is a formula about it and a lot of things come into it. It's a bit difficult to explain but in a nutshell we have some income sources dedicated to the burn, we have a natural burn system and then we have also expenses that come into the calculation. Putting all this together will define the burn rate of the week.
Our first priority is to insure stability and sustainability, meaning we need to cover our external costs like sever expenses. The second priority is to make sure that our accounts always have at least a neutral balance. Meaning we don't want to burn at the expense of something else. Finally, we want to burn as many tokens as possible to limit the supply of LEN.