TMN wrote:
....
But why do most plans on the net say to aim for about 4-5V OC ??...

I'd say it's because their secondary winding and tongs are "designed"
such that their resistance is too high and too much voltage is dropped
there. For instance,
http://www.5bears.com/welder.htm
uses #4 wire for the winding and brass for the tongs!! #4 is much too
small and brass is much too resistive. I'm impressed that it works at all.

Bob