To try to get some idea of how much current the raspberry pi wants to receive on its RX pin, I took some measurements with my (inaccurate) multimeter when using the mini PCB I just made:
Source voltage: 5.15V
Voltage drop across R1: 2.04-2.05
Voltage drop across R2: 3.10
(NOTE: our target voltage drop over R2 is 3.3 but the resistor values I used are not quite accurate enough for that; still serial I/O is working fine in my testing)
This tells me that current drawn by the RX pin on the pi is not affecting the voltage at all, and thus the values of 2200 and 3300 for R1 and R2 could probably be much higher. 1 mA of current is being wasted doing the voltage conversion using this quick-n-dirty approach.