Welcome. If your voltage sag is that huge, then something tells me you must be doing something awful wrong... either the reading is way off, or you are using a shitty battery, or your ohms are much too low, possibly it's a combination of these factors. What you call "raw output" is the resting voltage, which typically equals 4.2 volts if the battery is fully charged. The running voltage, or voltage that the battery runs at, equals the resting voltage minus the voltage sag. Use the running voltage for your Ohm's law calculations, but first, be sure to also understand how much voltage sag you will be getting from the battery you use. You can look at Mooch's pulsed current discharge graphs for that, and notice how the voltage sag is different depending on how many amps are drawn from the battery, and also notice how it's different for different batteries as well. Mooch wrote that these graphs will soon be replaced with number data. But for now, it's basically all we have.
If you are looking at a single 18650 battery tube mod, I recommend going for the Sony VTC5A battery (not to be confused with the VTC5).