Hi! It's been a long while since I've been active on here. I deleted my Dropbox years ago, so some things, like my Dragonfly Varileg guide, are lost to time. I may still have other things, though! Email me if you have any questions.
You can also buy Kazuo Haga's book "Origamics", which was recently translated into English. My copy arrived in the mail today, and I look forward to reading it!
You can use "guess and divide" it's accurate as much as you want.
general method:
step 1: guess where to divide the paper into two parts
step 2: divide the even side into two parts and forget about the old mark
repeat step 2 till it accurate enough.
in our case:
guess where to divide the paper into 6 | 7 - mistake X
divide the even side. now it is: 3 | 10 - mistake X/2
divide the even side. now it is: 8 | 5 - mistake X/4
divide the even side. now it is: 4 | 9 - mistake X/8
divide the even side. now it is: 2 | 11 - mistake X/16
divide the even side. now it is: 1 | 12 - mistake X/32
divide the even side. now it is: 7 | 6 - mistake X/64
divide the even side. now it is: 10 | 3 - mistake X/128
:
:
if your first guess was 10mm away from the correct position, at step 8 your mistake is less than 0.1mm.
BTW - if you started with 30cm paper and your first guess was exactly at the center the first mistake is 11.5mm