scientific Water change?

[Don Trinko]

You see recomendations for 100% several times a day on down to 30 to 50% every 2 weeks. Most of these recomendations are from respected Discus keepers. I know that Bioload,filtration and number and age of the fish all have some influence. It is hard for new Discus keepers to know what to do.
Is there a scientific way to determine when to change water and how much to change? Thanks; Don T.

[Ed13]

This is as clos to a scientific breakdown of water changes I've come across,and its right here in Simply
http://www.simplydiscus.com/library/water_chemistry/general/wc_formula.shtml