I've been assisting in the SDAA mirror grinding group here in San Diego and that's one of the first questions that gets asked. Don't worry about it. Use the first grit to get to about 110% of the radius and the rest of the grits to each halve that difference (10%) from the desired radius. The mirror will be hyperbolic growing towards shperical in shape due to the action of the grit until the last grit. Polishing the glass will get you all the way to spherical. Just make sure that the grit is pretty much distributed across the tool during the grinding and all will be well. The easy part is grinding as you can do any stroke and any speed and get a good mirror. The early grinding should have been at least 1/3 side overhang to get the most out of the grit. That makes the curve sharpen up fast. In other words, you don't have a problem.