The recommended CPU frequency is 1MHz, or alternatively the recommended mode of operation is ADC Noise Reduction Mode. This ensures good results from the temperature readings. Two-point calibration/compensation relies on accurate calibration points. If the ADC value at T2 is only accurate to within ±5°C, two-point calibration will only make the results less correct. However, when calibration points are accurate, two-point calibration/compensation gives good results over the whole temperature range.
Without accurate measurements at the second calibration point, one-point calibration is the best way to compensate the temperature readings. Offset removal using room temperature as reference gives ±5°C accuracy from -30°C to 100°C and ±3°C accuracy between 0°C and 85°C, all typical data for one-point calibration at room temperature for the Atmel ATtiny45. Gain error removal gives similar accuracy.