IMO classifying is for cons only, not for regular dirt less just dug from he ground. It just takes too much work. That is my lesson learned after five years of digging. Need to get something that will process your dirt to cons dug from the ground, otherwise it takes way too much time. In the desert southwest, its a dry washer. Dry washers tend to have a 3/8 inch screen built into them so you take the boulders out, and put the dirt on the screen.
Although I like the idea of 2 (1/2 inch), 4 (1/4 inch), 8 (1/8 inch), 12 --or-- 20 mesh, 50 mesh, 100 mesh, if you've ever actually screened a lot of material to this, it is hard and you'd have to be working with concentrates to make it worth your time. Really, any actual mesh set close to that will work.
One day, I decided to dig 10 buckets of 1/8 mesh material. Took me the entire day, sun up to sun down, to classify that many buckets. I went 1/2" to 1/4 inch to 8 mesh (1/8 inch) to run through the gold cube. I did it wet because if you don't, then you'll always get a bit of material sticking to the bigger material. Usually I classify dry, but that day I did it wet.
I ran these 8 mesh buckets through the gold cube on a different day, probably about another 5 hours set up to finish putting the gear away. I did not recover but a flake of gold. Not the gold cube's fault, but was my fault for digging bad dirt. I also dug around 40 buckets to get those 10 finished buckets. so I dug a ton of dirt, to get a flake of gold. Even if it had been an area you can take equipment into, if it was a gram a ton (about $1 worth of gold per five gallon bucket), then you have a serious pay streak you can take equipment into. I would have loved to gotten a gram that day, but got a flake. That was a lesson learned where I can't do the same thing the same way every time and expect different results.
Some like the trommels you can put on the gold cube, but I think they are way too small. Need to be able to shovel stuff into the machine by the shovel full, not with a green shovel but a #2 shovel, to make it worth your time.
Once you get the pay dirt, than by all means classify to what works at your claim. For me, I will take the cons from dry washing, and depending on the amount run them through the dry washer again to get "super cons" and then take that through 8 mesh directly to run through the gold cube.
Am I missing gold doing this? Yes, but not much. I have spent hours before getting every last fly speck of gold out of the cons, and I will never waste my time doing that again.