If you are building your own flat tracker, you need some measurements. These are the ones we're aiming for - measurements that have been collected from several sources and different kinds of flat trackers.
Wheelbase: 1375 mm
Ground clearance (height to centre of bottom frame rails): 200 mm
Seat height: 800 mm
Swingarm, from centre of pivot to centre of rear wheel spindle: 500 mm
Rake: 27 degrees
Fork offset: 60 mm
Ground trail: about 110 mm
There are of course many ways to skin a cat, but these measurements can at least be used as a starting point.