Analog read..... incorrect values as high as 5v... help please

And there you have a basic system.

That is the theory yes. Although a Zener charastic has a knee and voltage is not clamped bang on the knee voltage. I would use a lower value.

Implementation is another thing. Make all wires as short as possible, solder everything up, no Solderless bread boards. At this stage it becomes more of an art than a science