Figuring the amount of fabric for any given sofa or chair is not an exact science. There are so many different fabric patterns, sizes and styles of furniture, and skill levels of upholsterers that all affect how much fabric any item will take. At best, determining how much yardage something will take is a guess. Sometimes the upholsterer guesses correctly, and sometimes he can be way off. So, the trick is, to guess high enough to make sure we have enough fabric for the job, but to not have an excessive amount of fabric left over.
One more accurate method of determining yardage yardage is to go out and measure the furniture, then do a layout of all the furniture cover pieces, figureing in the sizes of pattern repeats, cut sizes and fabric width. However, this is very time consuming. Unless the client is will to pay for this service, this is not an option. Generally, the cost of the precise estimate would be better served in just making sure we guess high enough to have adequate fabric to do the job.
Pattern Repeats: Generally, many fabric patterns are based upon what I call "the rule of 27", which means that many patterns are created in some multiple of 27, such as 3", 9", 13 1/2", and, of couse, 27".