Help explain the math behind this high-side current limiting circuit
Help explain the math behind this high-side current limiting circuit

Through the action of negative feedback, the op-amp seeks to maintain the voltages at both its inputs at precisely the same value. Input +Vin is set by the the 3.3 volt supply and the resistor divider formed by R2 and R3. To make -Vin the same value (the aim of op-amp negative feedback), M1's resistance added to R4's resistance has to produce the same voltage at -Vin as that set at +Vin. So the op-amp manipulates the conductivity of M1 via the gate voltage until this happens (in microseconds normally).
Another way of looking at it is this; if M1's resistance added to R4's resistance equalled 3 kΩ, the circuit would become perfectly balanced (like a wheatstone bridge). This is because R1 and R2 are the same value. If R1 was only 3.6 Ω then the resistance of M1+R4 would have to equal 300 Ω to obtain perfect balance. It's the op-amp via negative feedback that drives M1's resistance to change and hence it produces the balance condition. Once the balance condition is obtained, there is no differential voltage input into the op-amp and, the circuit finds equilibrium.