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If you are going to raise then make it 200-250 to discourage mediocre over cards calling you.

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You're only going to get called by better hands if you raise to 200-250. 88 is fine to raise UTG, imo. 50*49/2 = 1225 combos. 99+ = total of 36 combos of overpairs. 9 handed table. UTG, 8 people left to act. 88 will be best hand on table... ((1225-36)/1225)^8 ~ 79% of the time. You shouldn't be varying your raises depending on the strength of your hand. You should, however, either vary your raises at random or make all your raises the same. My standard raise is to 150 @ this level. Don't be afraid to play postflop.