A general rule of thumb about pruning trees is that only healthy trees should be pruned. Pruning is a devitalizing practice that comes at the expense of the roots. If an avocado has root rot, make sure the tree has been treated with one of the phosphite products to get the root system healthy. A common pruning method is stumping to 3 feet and allowing regrowth to occur. A common phenomenon after stumping is that the tree puts on vigorous growth for two or three years and then collapses. All that canopy regrowth was coming from a large root system that was brought into balance with a smaller canopy. Energy is diverted from the root to fight off disease. Gradually the root system gets out of balance with a larger canopy that it can no longer support. Often when a severely impaired root system tree is pruned, it often does not have energy to push a new canopy and the tree dies. Make sure you only prune healthy trees.