how to calculate hysteresis resistor value TL431
Big thank you to all those who helped me get to the answer (See comments under the question)
Firstly as per recommendation the schematic was re-drawn to make it easier to read. There is a bit of an industry standard for this style of circuit I wasn't aware of.

To calculate the value R8 should be it was pointed out that it functions as part of the voltage divider and this divider value is different depending on the condition because R8 is either 0 or Bat+. Therefore the trigger voltage for Bat+ ends up being different for turning on the circuit and turning it off. Calcs to achieve values for a 2S lipo shown below.