Uh, you've gotta figure out the area of your pizza. Each inch a pizza gets bigger, it gains much more area (the circumference expands at a much greater rate.
Better way is to figure out the ratio in area.. In other words
pi(r*2) Or for your 12, pi(36) so about 113 sq. inches.
So, 425/113 = about 3.75 grams per sq inch...
a 16 inch pie would be pi(64) or 200 sq inches x 3.75 grams, or 750 grams.
It's not exactly a straight ratio-- more of the area is taken up in the flat section (middle) than the circumference, so you won't quite need 3.75 grams per sq inch, probably a bit less. I'd shoot for 675-700 grams for the 16 inch pie with your recipie.