The AFL Fantasy Magic Number
There is nothing magic about the magic number.  The only reason I can think of that it is called the magic number is as a label to refer to a common value used in a couple of formulas.
The simplest use is to calculate the approximate breakeven of a player that hasn&rsquot played a game in the season so far.  The player price is divided by the magic number the result being the approximate breakeven for that player.
The magic number is actually the dollar value of one Fantasy point.
The Preseason magic number
An approximate value for the magic number can be obtained by using the data from a single player. Using Brodie Grundy as an example he is priced at 6K and averaged 122.1 last season. Dividing 906K by 122.1 gives a magic number for Brodie Grundy of 7420.
However the magic number calculated this way will be different for players that have received a discount (10 games or fewer) or impossible for players without an average.  It will also vary for players that have not received a discount due to rounding.
To deal with these variations an average magic number is calculated using players that have an average from last season and have not been discounted.
Doing this calculation results in an average magic number of 7418.  Using this number to calculate the value of Brodie Grundy from his average (7418 x 122.1) results in a value of 5737.8  which when rounded to the nearest thousand is 6K.
We can also calculate what average a player is priced at.  This is especially useful for players without and average.  The number one pick from the 2019 draft Matt Rowell is priced at 0K.  Dividing this by the average magic number (270000/7418) equals 36.  So he is priced at an average fantasy score of 36.   This will also be his approximate breakeven score for round 1 if he plays.
The magic number during the season
The magic number is recalculated at the end of each round.  This is why the breakeven score is an approximate since the magic number from the previous round is used to calculate it for the following round. It is rarely the same but it is close so gives a useful estimate.
It is calculated using the sum of the values (before the current round) divided by the sum of the averages (including the current round) of the players that played in the round.
Since the magic number during the season is calculated from the players that played in a particular round it will most likely include players with discounted prices and immature averages (especially at the start of the season).  This will result in a magic number differs from the preseason average magic number which excludes these.