In my understanding, when you define the arrangement DRAID6 14+P+Q and using 1 Spare, you define that each disk will be "sliced" in 17 parts. Each part of the disks will be used in a rotated way, like the picture below.
Where S1..S14 are the slices of data, P and Q are parity slices and S are spare slices.
In this particular case, we have:
- 336x S1..14 - slices of data (24 disks with 14 slices of data)
- 24x P
- 24x Q
- 24x S
If you divide the size of disks by the number of slices in each disk, you will have the size of slice. In your case it would be 19.2TB / 17 = 1,12TB per slice.
So, in this case you should be something like:
- 336*1,12TB to data = ˜376TB,
- 48x1,12TB to parity ˜= 54,21TB or 2,8 disks
- 24x1,12TB for spare ˜=27,11TB or 1,41 disks
From this you can derivate the formula.
Obviously the numbers will differ slightly from the Modeller as the modeler calculates other things but this is a good way to explain the concept.