Are you compacting in horizontal layers or in inclined layers? I do not see the issue IF this is for granular pavement course. Max crossfall usually not more than 2%. I think that you are worrying too much "theoretically". Your compacted layers will not be 100% uniform in thickness. Secondly, sand cones would not "extend 250 to 300 mm. For me, I would just do a simple "normal" depth sand cone and get the results. If you are really worried, then scrape off a 100 mm or so and do another sand cone. If the material placed is a medium sand, say (river sand), then as a matter of course, you should scrape 100 mm or so below the surface and then do the sand cone. Due to traction, etc. the top, say 50 mm or so, might not be compacted to the same level. For sands, I like, if possible, to do the density tests on the previous layer.