Flat non-concrete subsurface for outdoor rubber tile
I have horses, we put down at least 4” of crushed quarry  or shale rock then use a plate compactor while watering it any low spots are filled and we get the areas quite flat then 3/4 rubber mats are put down. This usually holds up for about 5 years with even draft horses standing in the same spot (at the door with there head out) for hours a day our big boy weighs in at 1500+ 

If ours can hold up to this abuse it should be able to a person, a plate compactor will do a much better job than trying to do it by hand, add water and it really locks the crushed rock fines together. Note a compactor can be rented, also never use crushed river rock as it has partially round edges and never locks together like shale or crushed quarry will.