You can get a general idea about pressures with the 70° reading, but not that good. Better to test when the temp goes higher if you want perfection. Look at a range of 2 x ambient, to 2.5 x ambient. Typical is about 2.2 x ambient in a lot of conversions, in moderately humid conditions. The reason for testing in higher temperatures is that the pressure of R134a is not as linear in the higher temps as R12 was. This means you would see a lot of system cycling in lower temps where R12 wouldn't have cycled as much. That also means that the pressure is lower at lower temps than R12 would have been. And that it spikes in higher temperatures. The reading in the FAQ would be the low end of the pressure spectrum, but it is an effective reading if vent temperature output is good. And that is really the best judge of performance.

The ideal method of conversion is to have the pressure cycling switch reset to the low twenties. Around 20-22. Lower than that and it starts to freeze up the evaporator. You won't likely see the 30-40 reading on the low side at 60-70° ambient unless the car has been sitting in the sun for a long time and the windows are closed. Also, the higher temps are more enlightening when it comes to fan clutch and condenser fan activation and efficiency.