what side of the tank to read humidty

I keep a damp face towel over the screen top of the cold side and my digital reading reads about 15 degrees higher than the hot side. the cold side is a little high while the hot side is about right. I don't mist at all and the readings while the 50w inferred light is off is 80-84 on the cool side while the hot side is about 70. When the light is on it drops to about 74 on the cool side and about 50 on the hot side. The cool side I have a accurite indoor/outdoor with humidity from Walmart and the hot side I'm using just the analog temp/hygrometer gauge. Also its a 20L

I'm a little confused about what you have where... do you mean that you're reading the humidity at 80% on the cool side and 70% on the warm side?
Anyway, you want to know the humidity that accounts for most of the air in the tank. You might measure lower humidity on the hot side because the air can hold more moisture the warmer it is; so for the same amount of moisture, the relative humidity (which is the % of how much the air can hold before it condenses, at that temperature) will be lower. But that should only be true for the very immediate vicinity of the basking spot. The rest of the air in the enclosure should be at a fairly constant ambient temp; and anyway, a difference of 10° really won't make much difference in the % humidity.

So just go by what your Acurite says and ignore the analog gauge, which is probably wildly inaccurate anyway. Either way, you do not need 80% relative humidity for a BP.