I'm hoping that there exists a set of shortcuts for calculating TC when remaining decks is an inconvenient number like 2.25 or 3.5. It takes me too long to mentally divide, say, 17 by 4.5 - quick is it 3 or 4?

For example, with 1.5 decks remaining, I find it easier to double the RC and then divide by 3. Is there a set of similar tricks to deal with other divisors?