It has nothing to do with a scale system.  That would be the cats ass if a scale would talk to a controller.

This spreader will have a scale but just for reference. not to control the spreader.

I would really like what you said and why hasn't somebody did that?

I am useing a dickey john 30 GPM pwm off a liquid spreader we had for parts.  THe spreader comes standard with a manual 30 GPM control vavle.  I am not really doing something new here.