Table above is just a guide. Using the same wattage fitting with a narrow beam angle will help you light higher up the tree, while fittings of the same wattage and wider beam angle will not reach the top of the tree. Trees with darker foliage will not seem as brightly lit as trees of the same size, and trees with lighter foliage will look well lit. Trees in areas with less light pollution will appear more brightly lit than those in areas where there is more light pollution.
Trees with open canopies will allow the light to filter further up the trees, whilst dense canopy trees hold onto the light and make it harder for the beam to filter up to the higher branches.
Colour changing spots and floods can also be used to create dramatic effects. These fittings also have a white colour control which allows you to play around with different colour temperatures to give you the best white for your tree.