Smaller curves (your 3.5 diameters) are going to be hardest. I did a quilt with circles, and decided to redesign it using nothing smaller than a 5 or 6 inch diameter circle. but that was just me. Partial circles/arcs/curves at that smaller size should be more doable than full circles. Starch was definitely my friend, as it kept things from falling apart too bad with frequent seam ripping, otherwise, like the others said, match lots of points, whether that be all the apexes or half way/quarter way marks (not sure if you are doing long wiggly curves or quarter circles or what. Good luck, it'll be worth the effort when done!