Assume a pot of $100 pot if you bet half pot as a bluff you need your opponent to fold 33% of the time, if you face this bet you need 25% equity against your opponents likely holding.
1/2 = 33% = 50/(100+50) to call this bet you need 50/(150 +50) = 25%
2/3 =40% =66/(100+66) to call this bet you need 66/(166+66) = 28%
Pot = 50% = 100/(100+100) to call this bet you need 100/(200+100) = 33%
1.5 x = 60% =150/(100+150) to call this bet you need 150/(250+150) = 37.5%
2 x =66% = 200/(100+200) to call this bet you need 200/(300+200) = 40%