Section 7.5 of the datasheet for the SN74LVC1T45 shows minimum HIGH and maximum LOW voltages for output lines of the chip. Values are given for several different Vcc values (e.g. 2.3V, 3V, 4.5V). What's the most reliable way to interpret values for intermediate Vcc's? Or do you need to jump down to the next-worst-case value?
If, for some reason, you really need a value between those given in the datasheet, linear interpolation is the TI approved method for getting intermediate points. You might be saying "but not all those specs are linear" – and you'd be right. The fact is though, that across a small range (for example, between 1.65V and 3V), the variation from linear will be minor, and the datasheet values provide some headroom to the specifications. This method gives a safe approximation that is backed by TI and our characterization process.