Help with some pro farming tips?

I'm a mid-silver player, playing since the beginning of season 4, and my farming is at an average 150cs at 20 minutes. Sometimes I get those lucky 220 cs at 20 minutes, but high elo players can do that consistently. I'd like to be able to as well. I'm a midlane Kat main, and apart from missing less cs the problem I see is my roams make me miss a lot. Also I feel guilty taking other laners' farms. How could I keep up the farm and roam? Also, what is the exact best time to go B? After pushing out and the next wave is cannon? or? Any tips would be highly valued! {{sticker:sg-ahri-2}}
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