You did a really good job. About 3 feet is adequate. The reason is that the higher you go, the higher the temperature inside the compost is due to fermentation. The compost is often turned over to cool off this heat. However, if you stack the compost mix as low as 3 feet, you can complete it without turning it over. The fever rises and then goes down after about 15 days. You can use it when the heat of fermentation is almost gone.