Thanks guys

To figure out your base rate:
What are your monthly costs to pay for ALL your life and business expenses?
Assume you need to earn AT LEAST that based on four 20 hour work weeks a month. The rest of the time would be the typical unpaid work of maintaining your business, marketing, bookkeeping.

How much above the "cover all life's and business's expenses" depends on your skills.
That will determine your hourly rate.

Discuss and LOCK DOWN the specific needs of the client. Given your experience you may be pretty good at estimating the number of hours. Give him an ESTIMATE, NOT a flat rate. If he wants you to lock in the price than you lock in the hours. Include time for revisions in the estimate.

Make a production budget and include 'your fees' in it as crew member of whatever position you're filling. This becomes your cost price.

Over and above this cost price, you multiply X% (your margin). This can be calculated as suggested above. As long as you charge over the cost price, you won't be in the red (assuming you don't have running bills for your business that is).