You're misunderstanding how this is supposed to work somehow.
You take Miyuki's phone number, she changed the old emergency code (99905230523), and what you have to do is take her's by getting access to her phone, or even better, if you're at the rooftop scene, then you SHOULD have the right number via GOD's instructions. You take that number, and you SUBTRACT the keycode FROM that, and then add her variable to it. (Which would be 8.)
So to shorten this down more easily..
New God Number (-) 8 digit serial code (+) Miyuki Variable (number of creampies you unloaded inside of her, game will give you the number if you weren't keeping track.)