(elementary) standard line when scare card hits river?

[cbloom]

I feel like I'm losing a lot of money on this really common situation :

I have something like a decent top pair OOP, I'm leading the betting the whole way, about 2/3 pot. On the river my nightmare card hits - the flush makes it or whatever. What do I do?

My standard line on this has been to bet about 1/3 to 1/4 pot, but against aggressive players, that just screams "top pair, please don't bluff me!", and they will oblige and come ott and it's hard to call. The little bet is just giving away money.

How do I decide to check-call or even check-fold ? This is at party 6-max so I'll often see lower top pairs or even middle pairs here, so just folding the scare card seems much too weak.

[leehrat]

make a 45-50% blocking bet.

[cbloom]

Let me try some numbers.

Say the pot is P on the river. F fraction of the time they have the
flush, 1-F they don't.

If you bet 1/2P , they will always raise with the flush and you will
always fold. If they don't have the flush, they will raise anyway 25% of
the time and you will fold. So, F + 1/4*(1-F) you lose -1/2P , and
3/4*(1-F) you win +P .

EV = (F + 1/4*(1-F))*(-P/2) + 3/4*(1-F)*P

EV = P*[ 5/8 - 9/8F ]

If you check, they will always bet the flush. If they don't have the
flush they'll bet 50% of the time. Assume they bet 3/4P, should you be
calling or folding when they bet?

If you fold to their bet :

EV = 1/2*(1-F)*P
Ev = P * [ 1/2 - 1/2F ]

If you call :

EV = 1/2*(1-F)*P + F * (-3/4P) + 1/2*(1-F)*(7/4P)
EV = P * [ 1/2 - 1/2F - 3/4F + 7/8 - 7/8F ]
EV = P * [ 11/8 - 17/8F ]

if F is 0 (never has flush) the best line is clearly check-call
if F is 1 (always has flush) the bets line is check-fold

if F is 1/2 the EV's are

EV(bet) = P * (5/8 - 9/16) = P/16

EV(check-fold) = P ( 1/2 - 1/4) = P/4

EV(check-call) = P * (11/8 - 17/16 ) = P * 5/16

So the best line is check-call, but check-fold is very close, and betting is much worse

If there any F where betting is best?
Compare to check-call

[ 5/8 - 9/8F ] = [ 11/8 - 17/8F ]
F = 3/4

So betting is better for F >= 3/4

[2PAUL2]

check fold to a big bet is usually fine because they will check behind or bet small with the hands you are beating.

paul