Thank you for reporting this. Yes, the offset in the formula for the trend line should be in the range of 76..78. You can use the temperature level in he control loop for the tuning voltage input. Refer to page 6 in the AN305.
It seems that deltaVtemp in figure 8 is the subtraction of VTemp and some Vref. According to table 13 of BGT24MTR12 datasheet, there are two ref voltage: Vref_tx and Vref_lo, is one of these 2 signals used to make delataVtemp?