Base save bonuses increase at different rates for different character classes. In cases where multiple base save bonuses exist because of multiclassing, the most advantageous base save bonus for the category (e.g. will, fortitude or reflex) will apply, based on the applicable class and actual character level of that class.
The base save bonus is calculated as follows:
High: Class level / 2 rounded down + 2, Low: Class level / 3 rounded down.