Has anyone left a firm that now wants them to do consulting work for them? How did you charge them? How did it work out?

I took my salary and assumed benefits where 30% of it and worked out and hourly charge and thats what I charged my old firm. They were fine with it as soon as I explained how I got the number. One word of caution you may work more hours for free at your old firm because they were use to you working all kinds of hours but it is still worth the freedom of being out on your own.

I charged them like I did any other firm we usually bill by the project not by the hour

Don't forget to charge your out of pockets or they could kill you.