To reduce the confusion, I will call them Tiles (if you put 4 or 8 or etc. Tiles together you can get larger squares)
The equation I came up with is:
(F + 1)2 - 1 = T
Where F is the Figure and T is the number of Tiles.
Figure 5 : F = 5 thus T = 35
Figure 15: F = 15 thus T = 255
When T = 960 then F = (T + 1)0.5 -1 = 30