A couple things to consider:
1. What Type of field is PRCNT? I'd make a float. My bet is you have as an integer, and that type of field can't handle decimal places that you will have.
2. I always use Python expressions so it would look like :
!TOTAL_PROJ! / 1813828471
That just gives you a rational number. To get true percent:
(!TOTAL_PROJ! / 1813828471) * 100
That should just about do it....