On Earth, the Celsius scale was originally defined as scaling the difference between the freezing point and boiling point of water into 10^2 steps.

Avali would use Ammonia as the reference, as it is their biology's solvent and the most common liquid on Avalon's surface.

At the 151.9kPa atmospheric pressure at sea-level on Avalon, Ammonia has a freezing point of -77.73°C and a boiling point of -24.95°C, with a difference of 52.78°C.

Using a divisor of 6^2 yields a temperature difference of 1.466111111°C per Avali-degree (°A).

The conversion formula of °C to °A is thus °A = (°C + 77.73) / 1.466111111
And the other way around: °C = °A * 1.466111111 - 77.73

Freezing point of water: 53.01°A (125.002°A in base-6)
Boiling point of water: 121.22°A (321.115°A in base-6)

The Avali definition of the absolute temperature scale would also be different, so we also define a Avali-Kelvin (AK).

Absolute zero in Avali-degrees is -133.291°A (-341.142°A in base-6). This provides the offset of the °A scale relative to the AK scale. nAK = n°A + 133.291

Note:

A point could be made that Avali would use 6^3 to divide the scale. Although this will result in three-digit numbers being used to describe temperatures between the boiling and freezing points of Ammonia, this might be benefitial.
Avali, by using base-6, would already be used to handling numbers with many digits, but more importantly, the temperature range they can survive at is much shorter then that of humans. They might physically feel a temperature change of just a few degrees a lot more strongly then a human, and would thus be inclined towards using smaller steps in day-to-day life when describing temperature.