V8meathead, I know what you're doing wrong:
When using Thevenin to determine the current through Rs, you need to find an equivalent thevenin source for the rest of the circuit (the red block), this consists of Vth & Rth. For that you need to figure out the open circuit voltage & short circuit current at the terminals of Rs. 5mA is the SC current; it's not your final answer! Find the thevenin parameters as above & replace the red block with the thevenin equivalent source, then calculate the current through Rs, it is 3.3mA.
Ref: Nilsson & Riedel, Electric Circuits 6th edition 2001
Ah ha! Thanks, I appreciate it.