This is what I am currently doing...plus, since everyone loves a "deal", I charge a slightly higher hourly rate then give away "one hour FREE" because we all know that FREE is the most powerful word in marketing.

Basically, I figure out what I would be happy with for 5 hours for work and then give them 4 hours plus one FREE hour for that amount.

This has the added benefit of nudging my hourly rate higher, since I always have a hard time raising my rates.