Chris you're right with the errors 3-4 degC is a good estimate. Assuming it caps at 80% RH at 5degC the dewpoint will be calculated at 1.8C
I would factor in a small error band in this and turn on my dew heaters when I'm within 5-6 degC of the dew point just to be on the safe side.
Sidenote:
I needed the simplified formula because my dew heater had to handle powersuplies, USB interfaces, autofocus and a lot of other things so I had no spare microcontroller cycles to waste on doing a logarithm
Actually I was so tight that I optimised the simplified instruction to avoid dividing by 5 as well by pre-multiplying by 410 and then logically shifting right by 11.
The proper formula took 4678 instructions and 1658b of code (20% of the microcontroller's program space). The simplified formula was 407 instructions, and 268b of code. My final one was 74 instructions and 68 bytes.
