init_resize.sh : How to calculate ROOT_DEV_SIZE ?

[run_the_race]

I am trying to follow this unfortyunately locked thread: viewtopic.php?t=300187

How does one calculate ROOT_DEV_SIZE ?

From reading the init_resize.sh I think its specified in blocks (as opposed to the usual bits). When I try reverse calculate how people calculated it, I ways get none integer results, so I know I am incorrect. Say I have a 8GB SD card, what should I set ROOT_DEV_SIZE to use 6GB, and leave 2GB free (I wish to create a separate partition).

[RonR]

In init_resize.sh:

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  ROOT_DEV_SIZE=$(cat "/sys/block/${ROOT_DEV_NAME}/size")
  TARGET_END=$((ROOT_DEV_SIZE - 1))

ROOT_DEV_SIZE is the size of the device in sectors (512 bytes each).
TARGET_END is the last sector number of the device.

If you're trying to modify init_resize.sh so that it will leave 2 GB of free space at the end of the device, make the following change:

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  TARGET_END=$((ROOT_DEV_SIZE - 1 - ((2 * 1024 * 1024 * 1024) / 512)))