Canon GAFT requires actually rolling out the dice up to 12 dice. After that, you can multiply dice to approximate the sum of the actual dice if that works easier.
DICE METHOD
UP TO 12 12D6
13 3D6 x 1D6
14 4D6 x 1D6
15 5D6 x 1D6
16 6D6 x 1D6
17 3D6 x 2D6
18 4D6 x 2D6
19 5D6 x 2D6
20 6D6 x 2D6
21 3D6 x 3D6