the gas scale temperature is defined as:
T=273.16 lim Ptp->0 (P/Ptp)
The bulb of a constant volume gas thermometer is immersed in an ice-water-vapour mixture and the recorded pressure is 0.500 atm. It is then immersed in a boilng liquid and the pressure is 0.720 atm. Some gas is removed from the bulb and the experiment repeated, registering 0.250 and 0.350 atm respectively. Estimate, as accurately as possible, the boiling temperature of the liquid. 
The Attempt at a Solution
So here Ptp is recorded as 0.5 and 0.25, with corresponding P's of 0.72 and 0.35 right? Would it be better to take the average value calculated or use only the lower value of Ptp as it closer to 0? I'm a bit confused.