If heading E-W, (or vice-versa) longitude is the easiest way to give a position report.
If you are picking up the other folks on UHF only at the top of the dunes, then they probably are not too close (UHF is roughly line of sight). The lower down on the dunes that you can receive them, the closer they probably are. If you are getting them on the flat they are probably on the top of a dune within two or three dunes from you, so it's time to be alert.
That's how it worked for us, at any rate. We had a few folks give out good positions so we were ready for them. But others had no idea, and it was one of those where their flag was the first warning we had.
We also noted how far we were from the last check point (junction, lake or whatever). We kept a log of the mileage as we passed each point noted on our map. Might be overkill, but we never rely on just one form of navigation when out and about in remote areas. We never put our complete faith in electronics....
If you ever have to use your HF to call for help, and you've dropped, soaked, fused, or somehow buggered your GPS, then it's useful to be able to tell someone where you are by other means.
On the French Line and QAA line there was plenty of traffic. But down on the Rig Road we encountered one couple who said we were the first ones they'd seen for three days (and they were the only ones we saw that day). So it can get pretty quiet even in the holiday season.