Like I said twice, "It doesn't matter." The rule of regs or laws doesn't matter. Just get it off eBay and put it on the air. 433MHz is like CB radio ... wide open ... "engineers" just don't give a damn about the rules.
There is no "simple" conversion between (say) mW and mV/metre because it depends on the efficiency and impedance of the antenna, etc.. Thus you may transmit more "power" (mW) if you're using a poor antenna, or conversely less power if the antenna is directional.
A quick Google search found this link which appears to offer a conversion program. The introduction says:
"1MA85: Field Strength and Power Estimator
Determining the field strength from transmitted power is not an easy job. Various, quite complicated formulas have to be evaluated correctly. This application note explains, how to calculate electric and magnetic field strength and power flux density. A program associated to this application note helps to do the calculation and converts Watts to mW and dBm, V/m to mV/m and dBmV/m as well as A/m to mA/m and dBmA/m. Additional applications are calculation of propagation loss or antenna factors. This application is also available for iPhone, iPod Touch and Android Smartphones. It can be downloaded free-of-charge from the App Store and the Android Market respectively."
I tried downloading the Windows version, which appears to install but reports as "not a valid Win32 application". Perhaps somebody else will have more luck, maybe with Android or iOS.
I downloaded the field strength estimator for Android. Unfortunately, I don't know enough to use it. Based on the second image (HC-11 User Guide), I entered 433mHz, 10dbi, -110dbi, and 3 meters. I don't know if these are right, and I don't know what to do with the rest of the items, or exactly what I would expect to see.
If this is too far afield from Jeremy's original post, I'll delete this and start a new thread.
Indeed this may merit a new thread. Here in Aus/NZ we're allowed a paint bistering 25mW, so it's maybe best someone in US wrestle with their "regs." (or disdain of...).
I recall a previous discussion on the US/433 topic implied mere nanoWatts may be the US legal level, but licenced radio hams can run Watts.
EXTRA: More HC-12 just to hand here - any Kiwis readers fancy a play? Several already sent to Andrew Hornblow in Taranaki. Delivery took a mere 4 days after web ordering (at ~US$9 a pair freight free) from Satisfyelectronics .
On a practical level consider that the VERY tiny HC-12 size & it's supply tolerance means circuitry (including PCB & extra PICAXE etc) could perhaps be housed in a 2 x AA switched battery box. A single 3.2V LiFePO4 could power the lot!