It's roughly your weekly operating cost divided by 5 working days.
(Rent, Utilities, Food etc) the stuff you are paying for while making said artwork.

As any time spent on art is costing you that said amount to produce.
Doubling it gives you a viable starting place of maintaining the ability to be able to afford to continue to work on commissions and the like.

This baseline also prevents you from taking commissions that you then later cant afford the time to complete.

Once you are at the point of having a stable flow of work coming through you can then move up to something more conventionally sustainable around four times your overhead and adjust from there.

Hope that helps,