Almost dry enough to spread fertilizer

I have been attempting to spread fertilizer.
My goal is 80lbs of N per acre on the annual ryegrass. I am using 40-0-0-6.
The density of the fertilizer has varied from 48, to 52, to 50.
The chart for adjusting the spreader is somewhat confusing for me.
The neighbor told me I needed to use his spreader. He was right. His spreader is old but it has huge flotation tires on it.
The problem is that you have to maintain a constant speed and rpm in really wet/muddy conditions. And that it has a single spinner so you have to drive in circles instead of a back and forth.
I have a lot of trouble judging the distance.

Finally I went and got my GPS out of the grain drill tractor. I use FarmerGPS which runs on a tablet computer. You can switch GPS antennas really easily and it runs off a battery. It was not too difficult to program in the offset for spreading in circles with a single spinner.
It helped but it was hard to hold the GPS with on hand and drive with the other.

The problem with not really having any idea how many lbs I am applying is not so much the annual ryegrass, (Orin may disagree) Aside from the streaks in the field, at something like .27 cents per pound it really gets expensive.
I hear ryegrass straw will be worth a lot of money next year so perhaps my over application with make us rich!
Or not...