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It is important to note that LBA is a whole number, so any remainder resulting from the division should be truncated or rounded down to the nearest whole number. In addition, the first LBA on a storage device is typically 0, not 1, so the LBA of the first block is always 0. Typically, with an image from a hard disk, the block size would by 512 bytes.
You can then select the End LBA or the Block Count to set the number of bytes to be extracted.