So most of the time a character being created is given a number of AP and works within that. So if I have 40 AP and I want -8 DM, I can look at the chart and see that it costs me 16 of my 40AP. That's easy.
But suppose I am constructing a character the reverse way: I have 40 AP of stuff already and want to add -8 DM to it, how much is it? It can't be 16 AP--that puts the character total to 56 AP and then the cost is 22 AP.
What's the math?