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Winning rate

How many percent is considered as high winning rate?
It's up for you to decide, but i, personally, think that around 70%-75% is already a good win-rate
If you're playing similar rated opponents, you will score 50%.

This will be the same no matter your rating.

If you're playing lower rated or higher rated opponents, this will change, obviously.
It works in opposite way. If your winrate is 50%, your rating will become similar to your opponents. However, if your winrate is different for some reason, you may not get exactly 50% for equal opponents.
It's subjective and depends on strength of opponents as well.
But on general note (take any sporting tournament for example), winrate less than 40% is considered poor, winrate of 50%-60% is average, above 80% is considered high.