Wikipedia has numbers for Crr, and I have seen at least one guy's numbers from a coast down test on instructables. You can see how he did his test
here, and use his spreadsheet if you like. According to co_driver, you can include Cv (coefficient of viscous [skin] drag) also, and just approximate it at 1 Newton per Meter/second. You can see his equation in
this thread if you like. Like I said, you have to approximate the skin drag, since the solver function will blow the skin drag number way out of proportion.