Note that going from 1200 ELO to 1800 ELO is going from 18th percentile to 74th percentile. Pretty much any form of study over 6 months will get you that progress, because you'll have spent more time on chess than ~74% of players.
Going from 1800 ELO to 2400 ELO (74th percentile to 99th percentile) in 6 months would be a lot more interesting, because clearly your study habits are helping you progress faster than others.
A lot of being better than X% of people at something is just about spending more time doing it than X% of participants... Most professional video game players have 5,000 to 10,000 hours of experience in their game.