You can find the app here
CALIBRATION REQUIRES ROOT... PLEASE GO GET ROOT ON YOUR PHONE AND THEN COME BACK.
Oh and BTW... I cannot be held responsible for anything that happens to your phone.... EVER!
use "adb shell" or a terminal emulator to get a terminal prompt and run the following commands
Step 1. - Save old calibration value (Optional)
su mkdir /sdcard/S4data/ cat /efs/prox_cal >> /sdcard/S4data/prox_cal
Step 2. - Edit calibration value and update permissions
Grab the current value with everything clear of the proximity sensor.
Convert your calibration value to hex (The windows calculator works great for this... use programmer mode). Put your calibration value in place of the 96 in the command below.
su echo -en $'\x96' > /efs/prox_cal chown system:system /efs/prox_cal chmod 644 /efs/prox_cal sync
Big thanks to greaty for his post here