For dicussion purposes, TKN = Ammonia + Total Organic Nitrogen. It does not include nitrite or nitrate.
Influent TKN takes into account the generation of ammonia from biologial waste degradation by heterotrophic organisms. While some of the nitrogen is used in cell division and other metabolic processes, excess is converted biologically into ammonia. It is this ammonia that is used by your nitrifiers (ammonia oxidizing bacteria & nitrite oxidizing bacteria).
So when you are looking at ammonia removal effiency across a system, you should look at influent TKN as a better indicator of efficiency than using influent ammonia. Now in some systems influent TKN is 90% ammonia. In that case, influent ammonia is fine. If your TKN:Ammonia ratio is consistent, you can just run influent ammonia. Others with inconsistent organic nitrogen loads should run TKN daily.