Great question. Most consultants have some sense of an hourly rate.

However, if you have no idea, a very rough way to do it is take your salary divided by the number of working hours in a year. You probably then want to add in a multiplier since it is not full time employment. Extremely rough - but a starting point.

There are many other factors such as demand for your time, how much you value your time, your unique ability to help the startup, etc. that you can also factor in.