Generally, divide it by three and allow a 1/16th inch (or less) over hang on the front panel so that it easily lifts open to read it, your back panel will be just a little shorter in width as it typically folds into the center panel. Panels are basically; front=3 11/16", center=3 5/8", back=3 9/16" for an 8.5 X 11" sheet.
I have been in the trade for 30+ years, I have found that templates are okay but their development at times is done by a person with little or no actual application experience.