I would use resistor values that total roughly what the impedance of the headphones are, which is what the amp is supposedly designed to work with. Generally loading less (higher resistance) does no harm, but some amps may not have the specified frequency response without some loading.
There are various types of "headphones". The large ones that completely cover the ears are really small speakers and have low impedance, just like other speakers. The in-ear types can by dynamic, which can have a few 100 Ω down to speaker impedance, or piezo, which can have much higher impedance. If in doubt, I'd load a "headphone" amp with around 600 Ω. That should work well enough.
However, I would use substantially less than a gain of 30%. Microphone signals are a few millivolts at best, with headphone signals a volt or more. I'd try about 1000:1 voltage divider to start with, then adjust as needed. A good ratio is when both the headphone and microphone amps are at about 1/3 volume.
So if you need concrete numbers to start with, try 1 kΩ followed by 1 Ω to ground. Adjust the top resistor (the 1 kΩ) up or down by factors of 2 or so to get roughly the right attenuation ratio.