As a combine driver, i never empty if i think the trailer isn't under properly and again if the trailer is too full that it spills down the road then thats the combine drivers responsibility too in my eyes. The trailer driver has no control how much is put in his trailer, i just expect him to be under the spout when the lights are flashing.
I carted grain for a combine driver one year who was an absolute nightmare!! He was partially blind in his left eye and taking every load of the combine was a heart in the mouth situation. He hit my trailer a few times with the cutter bar :(
I'd say a fair bit of grain is lost when the spout is folded back in, always get a trickle out then and add all that up could be a surprising amount?! All the combines do it, new 570+ this year and cr 980 last year too.