When you're using an optical tachometer, remember that that the tach sees the contrast in light and dark areas in front of it. So, for instance, with your light colored (bright) wood prop, you ideally should have a darker background "behind" the propeller so that there is a greater contrast for the eye on the tach to see.

Sometimes, if you have a dark prop (like a Master Airscrew plastic) on a light background, remember that the sunlight reflecting off the blades can make the prop look "bright" to the sensor on the tach. If you're getting readings that are jumping all over the place, this is likely what's happening. Just try pointing at different areas on the prop disc or just turning the tach slightly to the left or right by 10-20 degrees or so so that the sensor isn't looking directly perpendicular to the prop disc. If the brightness of the prop is nearly the same as the brightness of background beyond the prop, that's when the sensor gets confused and you start seeing some of those wacky readings. When the sensor is seeing good contrast between the prop blades and the background, you'll have good stable reliable readings.