After programming Heat set to 70 and Cool set to 78...
When exactly do these functions kick in and turn off?
And what median temp can you expect... 74?

Perhaps this will help

http://www51.honeywell.com/pacific/common/documents/TH8000_VisionPRO_Use.pdf

I have both the user guide and operating manual. They don't appear to answer my question.

I`m not experanced with the th8000,but seems to me they both should cut off at the set point
Heat cuts off at 70
Cooling cuts off at 78

Not sure I`m understanding your question
Don`t think you can have them both set at the same time,for a median of 74 or any other degree

Your house will stay between 70 and 78. Over 78 the air will come on cooler than 70 the furnace will kick on, that is if the thermostat is set to auto mode.

"auto mode" is defined as "auto changeover"