I just plucked a number out of the air, rather than picking a number which I felt was representative of web development.

Also, I've harped on this a time or two, but there are people who get $2,000 a day for something which is indistinguishable from web development in terms of activities performed but which is sold as accomplishing business goals ("making lots of money") with incidental reference to programming.

P.S. Charge more for emergency engagements. The business stopping for a day is worth more than $1,000 even if the business is a taco truck. Don't do professional work for taco trucks.