26220

Problem solution

To work out the co-ordinates of any of the centre dots , you simply use this :
(3n-1, n+1)

for example the 20th centre dot would have co-ordinates of (59,21).
The 100 centre dot would be (299,101).

This sum is easily adjustable to work out the corners as well:
Top right corner = (3n,n+3)
Top left corner=(3n+1,n+1)
Bottom right corner=(3n-3,n)
Bottom left corner=(3n-2,n-1)