Can add some more advice.
It is not easy to manipulate such a tiny coin. If you are managing bare PCB (I hope so), and you need to remove more than a hair, take some bigger piece and superglue it to the PCB, this will give you more comfort in holding.
If you are tilting it too much and the sanding surface gets a little convex, you can both feel it and see it.
Find some time and make a small lapping place for such parts. Such lapping plates can be used to finish parts after sandpaper, or for checking them for flatness.
Find some piece (the flattest that you are able to find around) of metal. And a piece of glass (thicker=better). Glue them together with superglue or UV glue. Now you have a lapping plate with a flatness that is far over any flashlight modder needs.
DO NOT use any super abrasives (diamond or CBN powder\grease). They will stack inside metal but won’t remove any material. Do not start with polishing greases. Use regular aluminum oxide abrasives (find them in a liquid state or make your own from powder).
Once you start lapping, you can feel everything. If the surface is already flat, you will feel how big is sticking force (Gauge block - Wikipedia) between your plate and your part. Convex part does not stick to the flat surface. Also you see where is located freshly polished zone, and how big it is. After little practice you don’t need tons of towels around to clean parts every second stoke and see whats happening, you can feel whats happening with bare hands.
P.S. I do not use this technic for flashlight related parts