Keep cutting the header in 1/4" increments until you see a drop in RPM instead of a gain. Then, back the pipe back out that 1/4" and you are there. Keep in mind this length will change if you change anything such as plug, fuel, prop, etc. Even the weather can affect this. Because of this, most people cut until they see the engine starting to get peaky, then stop instead of going for the very last R's. With this approach, you can maintain pipe length over most conditions.

John