Mirror Polisher's Log Sheet mir-log.gif
You can use this sheet to plot the difference between a reference sphere and the actual curve of your mirror. Since we will be testing at 250 feet we will be using a reference parabola and plotting the difference between a reference parabola and the actual curve of the mirror.
OK, I can here the grumbling now. "The test star needs to be more than 3 million light years away before you can call it a reference parabola". All I can say is don't worry about it. If you can make a 1/10 wave elliptical mirror then worry about it if you wish.