As Christian has just pointed out, 0.228Hz/LSB is a better approximation, and I've re-computed as follows:
So this means the voltage control slope is 500/2750 = 0.228 Hz / LSB Converting this into ppb: 0.228 /.026 = 8.769 ppb / LSB
Based on this, the ideal value for AFC_SLOPE according to the measurements should be 229. I haven't tested that, this is just my thinking: 0.008769 * 32768 = 287
Also, to give you some idea what this means in terms of "Hz carrier frequency / LSB":
Center of GSM 900 between UL and DL: (960+880)/2 = 920 MHz overall multiplication factor for mid-band GSM900: 35.38 resulting theoretical slope: 8.07 Hz / LSB
Center of GSM 1800 between UL and DL: (1710+1880)/2 = 1795 MHz overall multiplication factor for mid-band GSM900: 69.04 resulting theoretical slope: 15.74 Hz / LSB