Well.....I gave the seeder a test run today before going back home.
I mounted the switch and rheostat on the left frame so that I can reach them from the seat of the 4 wheeler with one hand on the throttle. This way....everything stays self-contained. I can also reach the flow shut off. I have a small battery I use for my sprayer...and I will also use it for this spreader.
I put a few lbs of red clover in the hopper. This is about the smallest seed size I typically use and is similar to my brassica seeds in size. Seeds like this are normally applied at about 4 to 10 lbs per acre. I have the rheostat in order to narrow the spread for use on my trails and smaller plots which may vari from 12 feet wide to feild widths. The optional small seed plate may prove to be of value for trail use....and I may call Herd about this plate. Or else.....I am going to have to speed down the trails to get the proper amount put down?

I like to drive in low range at about 4 MPH for seed and fertilizer.
I adjusted the seed opening to about 1/8" (see pic of hopper bottom) and after some adjustments I was able to sling seed evenly both to the left and right. This required me moving the seed plate to about the "g" setting and was just a bit of trial and error to set up.
I found the rheostat will allow me to spread the clover from a minimum of about 10/12 feet to a max of about 32 feet (as measured with a tape). So.....every numeral change in the rheostat dial (from 1 to 10) equals about 2 feet in spread width. The L to R spread did not seem to be affected by the spinner speed settings of the rheostat.
The spinner rpm setting, ground speed, and seed plate opening, all need to be factored into your seed rate / acre. Once I get things dialed in.....it should be easy to replicate....I hope.
I ended up putting quite a bit of red clover in a small area while setting up. I think I will use some kitty litter to set up the spreader for fertilizer and for grains (rye/oats/wheat). Or maybe some low-cost grass seed....as I have an area to get seeded in grass too.
It seems like I should have all the needed features to sling seed and fertilizers evenly across my plots. The agitator seems to work well on seed and I am hopeful it does the same on fertilizer. I was a little disappointed that a pound or so of seed lays on the "floor" of the hopper as the flow becomes erratic near the end. (but then I'm critical?

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Note the longer bolts in the first pic....this allows me to remove the spreader and rest it on these bolts....rather than the motor. I had to use carriage screws as bolts didn't have enough thread to permit use....therefore I could not use stainless.
Heres some pics.