Actually, the sun at the front isn't the best thing for getting accurate readings. It's not about how much light is coming in. It's about the color contrast that the sensor sees. A bright white board or an overcast sky in front of the plane while you're reading the RPM from behind the prop will give you the most accurate readings. I know this from using a rifle chronograph which works exactly the same way, and that's what you have to do to them to get accurate readings (or any readings at all on some days).