I said this in the other thread, but just to clarify:

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Each tile is 16x16, the whole tileset is 480x256.

(480/16)x(256/16) = 30x16 = 480

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That's the tileset graphic file itself, not the number of direct tiles. The "grass" tile looks like 1 within the program, but it's actually made of 12 tiles - open a tileset in MSPaint to see.

In the game, the lower layer has 6x27, or 162 tiles, while the upper layer has 6x24, or 144 tiles. The total from THAT is 306 "useable" tiles.

Again, I really suggest opening up a tileset (after you make a copy of course). Just play around with it.