As the title says anybody know a good way to figure out how long and what angle a 12 bar should be on a custom subframe without putting the bikes nose in the air?

 

Back it up against a wall.
Length is a matter of opinion. I used to build them so the bike would sit perfectly vertical when the back tire was completely flat. Then with air added, the bike sits past vertical. This is key for stability if youre into scraping. Longer bars are easier to get to when youre new, but less stable when you get there.
For the angle, not much. I like about an 1/8" gap between the leading sedge of the bar and the pavement. Dont go crazy with this, because its gonna get bent anyway, and need to be bent back, probably many times.