I bet about $18 for 3 reasons
1. If they called on flop with a made hand, then they will call this bet still thinking their hand is good, so by betting $18 we can extract value from worse hands, then fire river again for smething around $40 since betting in odd ways such as there may look weak and induce the call from top pair
2. The flush draw, if villain was on a flush now, they just hit it, so by betting 18$ it seems like a weak bet so will definately induce the raise from villain either now or on the river. Also id re raise any raise, expecting hte flush to stack into us
3. Draws, if villain had a weird straight draw on flop or has now hit the flush draw on turn, we want to give them the right odds
to call to ensure we dont lose them, $18 should do this.
On river, id be putting in a large value bet but what size exactly depends on the river