MCUBoot Swap using move, how to calculate size?

We are using MCUBoot with MCUBOOT_SWAP_USING_MOVE enabled.

We ran into the issue, that we could not run the mcuboot upgrade, when the binary is too big (see also  Big primary image prevents FOTA ). The commenter suggest to reserve a partition of flash-sector size at the end of the primary image slot.

Also see the documentation of swap without using scratch here: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/mcuboot/design.html#swap-without-using-scratch

Do I understand this correctly, that we would require 1 sector of unused space at the end of the primary partition to do the upgrade? (32KiB on the nRF9160)

What I do not understand is, that it works fine with an image, where we have only 9828 bytes of free space at the end of the primary image. I would expect it to require 32KiB along the comments above.

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