First, you have to choose the usage of your eyeglasses, for seeing near/far/intermediate distance. Then, you have to enter the prescription for your Sperical Power(the value are usually showed as a positive sign"+" refers to farsightedness and presbyopia), Cylinder power & Axis (this value is available only if you have astigmatism), ADD=Near Addition, (important value for reading glasses)means the power strength to be adjusted with SPH number for making single vision reading glasses, computer glasses
, bifocal and progressive glasses. Some wearers may find their ADD value are put under the "N.V." on the prescription. Please enter the same ADD for OD and OS, if you have only one ADD value.
Learn more about how to read prescription at http://www.firmoo.com/help-p-27.shtml