Andrew.....very few have a robotic [3D] mind to sanding hand relationship, however a flat cork board for concave & a thin contoured cork board for convex profiles will always assist when sanding to achieve the perception of an ever changing flatness
on these surfaces
Naturally the material being sanded always varies in our visual perspective ...wood can change colour/tone as the contour changes & create sometimes false visual impressions of smoothness or flatness
Same applies to fiberglass surfaces............one trick to consider is as the trueness to final shape gets close ....spray the surface with a suitable undercoat & let dry....this helps in hightlighting any contour defect & provides a witness in the reclamation sanding process.....sometimes you may remove 80% of your first undercoat...& need to apply successive coats.......good luck....Derek