[R111S]

I wouldn't do it the way you're suggesting...or maybe I missunderstand.

1st...straighten the Rear Subframe as best you can.
It may help to have the frame bolted to the two unbroken tabs to discern alignment.
It may help to mount all the plastic to see if the lines flowing between front and rear parts line up.

2nd...mount the broken subframe tab to the Rear Subframe.

3rd...Once the rear subframe is straightend, position the assembly up against the Main Frame and mark the position of the broken subframe tab where it rests against the Main Frame.

4th...weld on the broken subframe tab.

My reasoning for taking these steps is that you may eliminate any stresses involved if everything is lined up in a non-stressed position before welding. Also you still have the two un-broken tabs to help.

As far as taking measurements from another bike...no two bikes are exactly the same...best to make yours work.