It's time and a half (1.5).  To my understanding only the .5 can be deducted.  That is why it's being divided by 3.  The check stub that I have has overtime at 1.5 (time & a half) and 200 (doubled time) When I entered the total amount of each one according to the this, the program calculated the deductible amount.

Overtime is 150 (1.5) and Double time is 200 

The W2 shows two entries - listed below

Overtime  

Double T