How do you measure for lampshade?

[momoftwins]

I need to buy a shade for a lamp that I have. I don't have the original shade, so I am lost. The distance from the base to the bottom of the socket is 23 inches and the body shape of the lamp itself is round. I don't know what bottom/top/height dimensions to pick.

Thanks for your help!

 

[golfgal]

I think the rule of thumb is the shade should be 2x's the width of the base.