The values of resistance in the regs are at 20°C, so to correct these values for 70°C then yes you would use 1.2
Resistance increases with temperature by 0.4% per 1°C from its 20°C value so an increase in temperature of 50°C (the difference between 20 and 70°C) would increase the resistance by 20% (0.4% x 50°C) hence you multiply the 20°C resistance value by 1.2.
If you want to correct measured values then you would need to know the temperature at the time you measured them.
Let's say the ambient temperature was 25°C when you took an R1+Rn measurement. This time the temperature difference is 45°C (0.4% x 45°C = 18% so multiply by 1.18).
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