Here is one link. There are many links on the internet so you may find one you like better but this is pretty decent.
http://www.turningriver.com/texas-holdem-drawing-odds
As you will see in the link the odds of catching your gut shot with one card to come is 10.5 to and with 2 cards to come it is about 5 to 1.
You would use the 2 cards to come to see if it is worth staying to the river from the flop on. Generally you need to make that decsion card by card so the 1 card to come tends to be more useful in my opinion.
The simple answer to your question about needing 22.5 to 1(2 card) to vs the 45 to 1 ( 1 card) is that with 2 cards to come you have twice as many chances to hit.