There are two kinds of far-field measurement: gated and not-gated one. In the gated measurement we set the markers so that we limit the data to be used to calculate the frequency response. The not-gated far-field measurement, instead, uses all the data including the room response, and is useful to see how the speakers will sound in the environment they will be used, helping us understand where to place them.

In the far field measurement the microphone is placed at a distance at which the driver looks like a point source and its sound pressure level is inversely proportional with its distance from the mic (6 dB drop for each doubling of distance); the industry standard sets this distance to 1 meter to allow integration of the various drivers in a front baffle, that is by 1 meter usually the woofer and tweeter response have added together so a single test gets both (however when testing speakers that uses line sources, like ribbon or array drivers, the mic-speaker distance is increased to 2 meters). If you are testing drivers separately then you can use any distance that works but be aware that if it's true that a less than 1 meter distance increases the signal to noise ratio, it's also true that a too close distance causes the driver not to act as a point source, thus the measurement cannot be considered far-field. Is common to use a distance equal to 4-5 times the driver diaphragm diameter, but not exceeding more than 1 meter: it rarely works for anechoic (gated) measurements cause the reflections are very close to the signal and you don't get any low frequency response (unless you have a huge measurement room, where floor-ceiling-sidewall reflections happens very late). I suggest you to measure the DUT at various distances, for example at 60–80-90-100 cm, and compare the responses: when the curve shapes are similar means that you are in the far field, thus that is the right microphone-baffle distance. Using 1 meter or more for far-field (not gated) in-room response is just fine.

In the gated measurement it is very important to place the DUT in the room center, so that all the parallel walls are equally far from it; I use to put the speaker on a table to center it between the floor and the ceiling. Also put some absorbing mat on the floor between the mic and the speaker, as well as on the microphone stand. Here's some pictures:

Let's start with the gated far field measurement !