Rew,on existing instalations, if you downsize more than one size you may run into air flow problems unless the blower cfm is the same because of duct sizing.As far as load calc, once you become confident in the equiptment size that the program says to install you need to be able to explain to the HO that the furnace or air will run a lot more than their old unit did but will cost less as a result of using smaller equiptment.But they will be more comfortable because everything is sized correctly.