A luthier I know told me 40-45 is ideal for most guitars, but most can handle a range a bit bigger than that. Older Takamine guitars seem to be quite robust, but Martins not quite to the same extent.
Are you sure that 60-80% relative humidity is correct? That's beyond most rain forests!!! And there usually is a very large variance between summer and winter, but it would depends on what state you're in.
If you're looking for humidification, Planet Waves (D'addario) make a good one that rests between strings, sitting in the soundhole. Can be used with or without case, and is very inexpensive. (About $10 methinks?)