@Twitch.http://www.thctestkits.com/ check that website out I think they use the same method as the one you listed. The entire process is on that video it shows the method of analysis.
20% is not high for an extract. When you do a separation of cbd its done very specific mechanically w heat, like a scientific melting point apparatus kit and melt close to, but not over the specific crossover temperature and melt and spill the extract in another container. Allowed time to reach its target temp and one can get 50% cbd fractions and also thc fractions easily and repeatably.