Over the years I've often read where people had difficulties changing the bobbin in a machine that takes a vertical bobbin case. I never understood that as I've never had an issue with any of my machines. My mom and I were talking about this one day and it finally dawned on me - my mom is right handed and I'm left handed. IMO it's only natural that a left handed person would have an easier time inserting a bobbin case because of where they go into a machine.
I basically do what WIChix does, hold the lever out with one finger while putting the bobbin case into position, just going by feel that it's in place. Once it's in place I let go of the lever.
Cari