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A1Ckot, trust me when I say your list of questions about how to use that fixture would be pages long if you actually bought one. But to answer your specific questions, a brake bridge position would be measured with a metric ruler from a rear axle and held in place by the friction of its miters. The method is file a bit, measure, file a bit measure and repeat on each side until it is the right length and the hole is centered. There are various methods to make sure it isn't crooked. The bb and seat tube are brazed together first and don't need to be in the fixture for that. Chasing, facing and reaming is done afterward except for the faces of the bb shell.
I remember chatting with Ben Serotta at the New York bike show way back in the day when he was just a single man operation like me. He said one of the reasons we like to become framebuilders is because we enjoy designing fixtures. Over the years I've spent hundreds if not thousands of hours thinking out how they should be made – often with other engineers. There is a laser cutting company specializing in stainless steel that makes mine.
I suggest that if you are serious about ever making a frame for yourself that you take a class from either me, UBI or Dave Bohm. At the very least you would end up with a frame built for yourself. I've been teaching these classes ever since 1976 after I got back from apprenticing in England. In fact my reason for going in the first place was so I could get the knowledge to be able to teach Americans so they wouldn't have to trek abroad to figure it out. Organizing and presenting how-to-build frames has been my primary life's work.
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