As far as I know it's all based on data generated by 7Sage's analytics. I'm more certain it's the case for the percentages than for the difficulty ranking as the latter could have been assigned by JY et al.
Tighten up that LG, and then use the analytics to really figure out what question types on LR are really hurting you on times tests. RC should improve with LR improvement and understanding so just keep plugging away.
This message is for all Canadians: If you only need a 160, just keep PTing and build up your analytics, drill the problem areas, and make sure your LG skills are on point... that is all.
You can also "star" questions you miss on PT's using 7sage analytics ... This is HUGE to me as I can simply select all of my tests and sort by the stars! And then target them with my intellect.
Don't touch a PT until you finish the curriculum... Then take 3-5 PTs, check your analytics and adjust fire from there. Doing them before you finish the curriculum is what we call "wasting PTs".