The equation "Temperature = (ADCCODE/32) - 256" is an approximation that works mostly for temperatures in the range of -100C to +100C (error of -1.75C at -100C and -1.4C at +100C). For a more accurate reading, it is recommended to use the Callendar-Van Dusen equation found in the "Temperature Conversion" section.
According to the RRTD equation in the "Converting RTD Data Register Values to Temperature" section, there are 82 steps for each 1 Ohm variation [(400*82)/32768] when using a PT100 with a 400 Ohm RREF. The ADC steps are directly linked to the RRTD steps, which can be converted to temperature using either the lookup table or an equation. Referring to the lookup table represented in Table 9, then there are 32 steps between each 1C change.
In conclusion, understanding the relationship between the ADC code, temperature, and utilizing the appropriate temperature conversion equations will help in obtaining accurate temperature measurements with the MAX31865 and PT100 RTD configuration.
