Shank to base question

[Rotsco]

Hello all, I was hoping I could ask if anyone could give me some advice on what is probably a non-issue for most of you.

Lately, I've been having trouble attaching my ring shanks to the base and keeping it 100% perpendicular.
I believe that the majority of the problem is coming from when I file off/down the shank to allow for soldering to the base.

At the moment, I hold the shank in a ring clasp, then hand file by eye, trying to keep it all straight and true.
Which, for me, is hit and miss.

Is there another way I could be approaching this? Or should I just keep trying to improve what it is I'm already doing, so it's more hit that miss.

I'll include a little diagram to help explain..