SuchTileRender.cs

// World.cs
public Rectangle GetSourceRectangle(Texture2D tileset, int tileIndex, int tileSize)
{
	var borderSize = 1;
	var paddedTileSize = tileSize + borderSize;
	var tilesPerRow = tileset.Width / paddedTileSize;
	var x = paddedTileSize * (tileIndex % tilesPerRow);
	var y = paddedTileSize * (tileIndex / tilesPerRow);

	return new Rectangle(x, y, tileSize, tileSize);
}

void CreateTiles()
{
	var tilesheet = content.Load<Texture2D>("tilesetTest");

	for (var x = 0; x < tiles2D.GetLength(0); x++)
	{
		for (var y = 0; y < tiles2D.GetLength(1); y++)
		{
			var typeNum = random.Next(0, 200); // 200 seems to be some odd seed value???

			tiles2D[x,y] = new Tile(this, tilesheet, typeNum);
		}
	}
}

// Tile.cs ctor
sourceRect = world.GetSourceRectangle(tileSheet, type, 48);