Actually Nall has only completed more than 50% of his passes in 9 pre-season game, against the Falcons and Browns in 2003:
http://www.nfl.com/teams/schedule/GB/2005 (only played in the Buffalo and Tenn games)
http://www.nfl.com/teams/schedule/GB/2004 (only played in the Jags and Tenn games)
http://www.nfl.com/teams/schedule/GB/2003
The only time he saw action during the regular season was in 2004, but that was in garbage time of a blowout win and loss.
JP went 7-11 with a dropped ball, for a 63% completion rate (or 73% if you count the drop). Can't really expect much more than that.
And reportedly tonight he was 15-26 with 3 drops.