The conversion rate in Odyssey is being calculated as follows:
First, we take all the sessions that a traffic source is responsible for (e.g. 100). Then, we look at the number of converting customer journeys that this traffic source contributed to (e.g. 10) and divide this number by the total amount of sessions (10 / 100 = 10%).
This is pretty straightforward, but there are a couple of tricky issues here that you have to be aware of. Because Odyssey is a multi-touch attribution tool, there can be multiple sessions from the same traffic source that contributed to the same conversion (e.g. there could have been 20 sessions from the same traffic source that contributed to 10 conversions). Also, different traffic sources could have contributed to the same conversion (e.g.: These 10 conversions are also being counted in the conversion rates of other traffic sources; therefore the conversion rates are higher on average than conversion rates based on single touch attribution).