Using the FS_XL = ±8 option, the sensitivity is stated as 0.244. To convert from mG to m/s^2, the calculation ought to be as simple as the following:
acceleration * 0.244 * 9.8 / 1000.0
Bear in mind this worked perfectly fine with my BerryIMU V2 using the LSM9DS1, albeit with a different sensitivity value. But now it does not work at all.