Homeschooling books don't agree on the value of a credit. Some books say 120 hours, and some say 150 hours, and some say 180 hours. Some say could a "hour" as just 20 minutes. Others say an "hour" has to be the full 60 minutes and must NOT include homework (which I find hilarious, since ALL work for homeschoolers is homework!)
Based on that information, I think you can pretty much just DECIDE on a scale for a credit, and then stick to it. Parents know best!
Learn much more about how to keep high school records in my “Preparing to Homeschool High School” DVD set.