To achieve accurate temperature measurements with the MAX31865 and a PT100 RTD configuration, it is important to understand the relationship between the ADC code and temperature.
Please see the equation "Temperature = (ADCCODE/32) - 256" is an approximation that works primarily for temperatures in the range of -100°C to +100°C, with some error margins at the extremes.
For more accurate readings, it is recommended to use the Callendar-Van Dusen equation found in the relevant sections of the documentation. Additionally, there are 82 steps for each 1 Ohm variation when using a PT100 with a 400 Ohm reference resistance, and the ADC steps are directly linked to the RTD steps, which can be converted to temperature using either a lookup table or an equation.
In a 4-wire configuration, only one excitation current is required, which simplifies the design. Three analog pins from the ADC are utilized: one for the excitation current and two as a fully differential input channel for sensing the voltage across the RTD. This setup will help ensure that you achieve the most accurate results for your temperature measurements.
Please see the tutorials/links for a deeper understanding.
Understanding Temperature Measurement with MAX31865 with a PT100 RTD
How to Select and Design the Best RTD Temperature Sensing System
How to Select and Design the Best RTD Temperature Sensing System