As per the header, how are cold cranking amps calculated? Im trying to understand what a rating of 20AH means if I want to use it as a starter source.
Cold Cranking Amp is a measure the ability of a fully charged new battery, in a cold environment (0°F/-18°C), to discharge a load (in amperes) for 30 seconds, while still maintaining a voltage equal to or higher than 1.20 volts per cell.
It sounds like you are describing a deep cycle battery. Don't confuse a deep cycle battery with a starting battery. They are quite different. A deep cycle battery, rated in AH, usually cannot deliver the burst of energy needed for starting an engine.
That 20AH will not tell you about the cranking capacity - you need to find the cranking capacity for the battery you are looking at - this is given by the manufacturer usually at a specific temperature.
The 20AH means the amount of current the battery can deliver : basically 20A for 1 hour (=20AH), 2A for 10 hours, or 1A for 20 hours. However, this is more a guide and not exact.