Hi you can find the phone sweet spot within your nfc phone hand book (user guide).
so here is the xperia sp phone:
To turn on the NFC function
From your Home screen, tap Application screen icon in Sony Smartphones . Find and tap Settings > More…. Mark the NFC checkbox.
NFC detection area
Nfc detection area on Sony Xperia™ SP
The location of the NFC detection area is not the same on all devices. When sharing data with another device using NFC, refer to the User guide of the other device for more information.