If you make one, and the scriber isn't centered perfectly, just reverse it and scribe again. The center will be half way between the two lines you make.
A friend of mine scaled it up so the pivot pins are about 4 inches apart since he works on wider stock.
In order to get the holes centered, I made a drilling fixture and have made dozens of these using it. But, the hot rolled stock that I have been using isn't all that accurate, so I have to be careful. Cold rolled stock would be a better choice.