Here's what we use. First scale system we installed on a BBI 14' spreader. Scale-Tec sysetm. Worked great.

Traded BBI for an Unverferth Pro Force with factory installed system. Hooked our Scale-Tec POINT monitor unit to it. Also it reads through the Ravens RCM going thru ISO going to JD 4640 monitor. Works great.

I calibrate spreader by loading product into spreader. Setting spreader for FIXED RATE of whatever....like 200#/A, not VR. Make sure monitor has same spread width as autopilot steering. Then spread a few acres. Note amount used. Compare to what should have been used. Adjust as your spreader OM suggests. Either adjust belt speed, gate opening, or product density to get near perfect. I find also easy to check & verify the product density using those handheld sliding scale things. Now you're good for fixed rate. That's as good as it's going to be.

In my limited  years of experiences with VR anything, you'll either be lucky or lying if you put on the field exactly what is called for in the VR prescription. Too many variables....but it's the best that can be done for now. But again, if spreading fixed rate then you should be able to come within a few pounds of consistency. I also think you'll find that even at a fixed rate, the accuracy between 5 mph, 10 mph, and 15 mph will be different. I have come to realize the custom guys are much better operators and mathematicians than I because I know they always used the amount that was called for in the field. I haven't been able to hit that mark yet. I guess it's one of the downfalls of doing your own stuff.  Another similar point....I also note the product density I find with my handheld scale is different than most suppliers quote to us. I just go ahead and use what I see on the scale.

That's "here" anyway.