There is another way, but it may be difficult to make the measurements. There is a device called a spherometer for measureing the curvature of a surface, but you probably do not have one of those laying around. You could accomplish what it does if you could make very accurate thickness measurements with a micrometer or vernier caliper, and if your lens is symmetrical (same radii on both sides). A very accurate measurement of the thickness of the lens in the middle, and at a known distance from the center (25mm) can be used to find the radius of curvature. Most thin lenses are not knife sharp on the edge, so you might want to move in a bit and make an actual measurement just inside the radius. Is this a possibility?