As I previously mentioned, I don't use charging for amounts of working hours (Hourly rate). I don't work as an assistant anymore (Sketching, Inking, coloring and/or lettering) but as solo lead artist, so I do everything myself.
The average minimum I mentioned is for a coloring assistant (Freelancer / Independent) who does all that, that means flat colors, gradients, shading, etc. Depending on the person hiring, some may charge the same amount for just flats, but its a matter of searching and finding. Most people making webcomics are either artists who do everything themselves, so most cannot afford an Editorial like type of payment for the extra help they manage to get.