How do you charge for your art?

I often get asked how to charge for artwork. I have a simple formula:

materials (canvas, paints, etc) + time @ £50 per hour + £10 per artwork for studio costs (my studio, advertising etc) + additional fees (gallery or commission fees) = price

A long time ago someone told me that if you feel that you have charged reasonably, honestly, and given value for money for your skills and expertise then you are charging correctly. other advise, cheaper doesn’t necessarily mean it will sell, people will undervalue you if you undervalue yourself.