I'm puttiong a wood stove in my shop and running the chimney out the side wall of the shop on the south side 8 ft up the wall

Normally the chimney the needs to be as high as the peak of the roof to prevent downdrafts into the fireplace Straight line wind blowing across the peak of the roof will dive as it passes over.

There is no hard and fast rule but in practice, the end is placed 2-3 ft above the ridge. Also, make sure to put a deflection cap on it. A gas vent cap prevents water from coming in and keeps downdrafts out. Otherwise, you will have smoke backing into the firebox and your shop.

There actually is a rule, a chimney must not terminate lower than 2' above any roof point within 10' and it must be at least 3' taller than where it exits the roof line, called the 2' 10' 3' chimney rule.