After thirty years in engineering, and then 3 years retired, I have been asked to act as an engineering consultant for one of my old customers. Obviously there are all sorts of pitfalls, including liability if things go wrong, but initially I am worried about setting the correct hourly rate.
Actually I was wrong about my time in engineering. I worked for the same company for 30 years, but was in engineering for about 15 years before that last employer.
Have they offered a day rate yet?
Do you know any other engineers in similar positions who you could ask?
Are there any recruitment agencies let looking for contractors in similar positions?
Those are the kind of places to look to guide you on what rate you should be going for.
Or you can aim for a day rate 50% above what you full time salary equivalent would be.
Hope that helps!