I think a surface gauge (like the Grizzly one Scooper posted a picture of) is just the right tool to use with a flat table. Bicycle tubing is inaccurate enough that putting a dial indicator on a post with .001" graduations is just a recipe for driving yourself nuts. A tube just isn't accurate enough for such fine gradations. Listening to the sound of the tick of a point scratching the top of a tube has the right amount of accuracy for not-perfectly-straight tubes.

If I was to start out again, I would buy a flat table first before I got any other complicated piece of equipment (and I've acquired a lot of equipment over the years). 2' X 3' if I was space and financially challenged and just under 3' X 4' if I could swing it. That way I would not be dependent on a fixture for alignment accuracy. However I held the tubes to match my design, I would use the table to get a straight frame.

Just as a point of reference, all my framebuilding class students can get a frame within a mm (or greater) accuracy on their very first one. If you are taught the right sequences of tacking, aligning and brazing, and you choreograph your brazing patterns correctly, they will end up to be straight. If your 3rd frame is crooked you either weren't taught right or you didn't learn right or you were sloppy.