The UEFI specification is not ambiguous - you should always look
for the backup GPT Header at the last LBA:
"Two GPT Header structures are stored on the device: the primary
and the backup. The primary GPT Header must be located in LBA 1
(i.e., the second logical block), and the backup GPT Header must
be located in the last LBA of the device."
If the primary GPT Header is corrupted (e.g., CRC is bad), you
cannot trust any fields in it, including the Alternate LBA field.
The Alternate LBA field is there to help you tolerate failures
while growing or shrinking the block device size (not important
for individual physical drives, but an issue for logical drives
presented by RAID controllers).