I did a bit of calculations, and I just realized I gave you enough dirt to complete 1/9th of the island.
Math to Support
Total Blocks Needed=22500
(150x150)-(64x40)=19940
22500-19940=2560
22500/2560=8.7 which rounds to 9
Don't question it, I know there are much simpler ways to figure it out but this way gives the amount of blocks needed as well, which brings to mind
MORE MATH
19940/64=amount of stacks needed
Amount of stacks needed = 311.5 stacks
Amount of double chests = 5 and a bit
311.5/60=5.2