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a) It depends on your skill relative to your opponents'.  In general though, something in the vicinity of 10bb/100 is doable at normal tables, and maybe a bit lower at zoom tables.

b) Hard to comment without knowing anything about your game.  You might want to go through your PT4/HM2 database and figure out where you are losing the most amount of money.  It is possible that you are leaking in 3b pots, but that's just my guess.

c) There are many.  To list just a few, they play too loose preflop, and call and pay off too often postflop.  The latter is usually a consequence of the former.

Also, post some hands in the forum!  Usually that's the fastest way you can get help and improve your game, based on my limited experience.

I have been playing ZOOM PLO50 since the beginning of the year. I have made around 4bb in the last 100k hands but I found some huge leak in 3bet pots beginning of this month and have been able to make 11bb/100 with an EV of 14bb/100 of the last 30k hands! (not a big sample but not bad)

I think anything between 5 and 10bb is reasonable if you play well and play an exploitative way against certain opponents.

Most important is positional awareness and to have tight range in EP(espeically play MW hands) and loosen up from CO and BTN. Work on 3 bet pots and when to 4bet or maybe 3bet AA hands and gii on which kind of flops. This is were most of the money is so it is important to play those spots well. 

Biggest leaks of players is def their positional unawarness!!! They play same stats from all pos.

They dont defend light IP vs. Steal or 3bet. Tight with stealing blinds from the BTN. Stack off too loose in MW pots.

This is just more of a general answer but I think you get the idea!

Just to make things clear: The winrates in above posts (like 10bb/100) are way way way above average, or even exceptional. Most regs dont have anywhere close to these winrates. In fact, most regs in these games make somewhere between -1bb/100 to +3bb/100 pre-rb (plus rakeback of around 3-3.5bb/100).

So obviously try to aim for as high as possible (10bb/100 or even more is definately possible), but dont let yourself down if all you can achieve is +3.5bb/100 pre-rb, because even that is pretty good compared to most regs.

Well 3bb is definitely good especially before rakeback.  I dont think that a lot of ppl are making 3bb at those limits. Even the regulars that I play with everyday have some big leaks cuz otherwise they would be playing higher! Good players dont stay at those limits, winnings per hour are too low and rake is so high!

Exactly! If everything goes well I will be playing PLO 100 in June and no more grinding in the 50s! :-)

I already played PLO50 at the regulars where I have like 15bb/100 over 200k hands. My pretty expensive shots at PLO100 failed terribly and now I am back to PLO50 rush tables where I made 19bb/100 over the last 20k hands. But I am pretty sure I won´t hold that, but 10bb/100 just alright to pay the bills and try another shot next year.