The acronym TIR means total indicator readout, so is the measure from low to high. It could be double the runout of either side from center if the deflection is symmetrical, but remember that one side may be where it belongs and all the error on the other.

One way to look at this is to imagine spinning earth. The highest mountain is 29,000' above sea level, but if you were to run an indicator on Earth it would bottom at Challenger deep at 36,000' so the total TIR would be 65,000', and the sea level not the mid point. TIR is useful as a measure of error, but if you were truing Earth, you'd do more correction on the depth side, than at the mtn side.

BTW- question, if, with global warming polar ice raises sea level, does that mean that all the mountains just got shorter?