How much luck and how much intention?

The first couple times I saw this movie I wondered if Tom was planning ahead or if he just threw his "loyalty" to whoever was on top at the time. I've settled on the conclusion that he was more or less loyal to Leo the whole time. I don't think he had any far-reaching plan, but rather, was reacting to events as they occurred.

With regard to the original question, I think he was very, very lucky that Bernie killed Mink and left him in his place. After that, though, he was clever enough to figure out how Eddie Dane's blindness to Mink's betrayal (he was tipping Bernie about Caspar's bets) would lead Caspar to believe that Dane had betrayed him.