A method of approximating our actual advantage is required for betting and play variation. The most popular method is to use the count per deck method, which is called the true count. To calculate it, the running count is divided into the number of decks that have not been seen. In the first example above, a running count of +5 with 5 decks remaining gives you a count per deck of +1. In the second example, a running count of +5 with 1 deck remaining gives you a count per deck of +5. The remaining cards in the second example are much better than the first. Therefore, you should always calculate the true count before making betting or playing decisions.
Some systems use different methods of making the calculations. As an example, you can divide your running count by the number of half decks remaining. For example, if you have a running count of -4 with 2 decks remaining, your count per half deck would be -1 since there are 4 half decks that you have not seen.
I have found that it is easier to calculate the count per deck rather than "per half deck". As a result, the numbers for bet variation and insurance quoted in this guide are based on a count per deck.