The auto disk powder measure uses a system called Volume Metered Density (VMD) to dispense powder. The cavities in the disk have a number assigned to them which is actually the volume of the cavity in cubic centimeters. The powder companies provide a VMD number or value for each type of powder. If you multiply the number of grains you wish to dispense times the VMD value the answer is the theoretical volume in cubic centimeters that are necessary to hold that grain weight of powder. Or grain weight x VMD = cubic centimeters. You can find the Powder VMD's here: http://www.leeprecision.com/cgi-data/instruct/VMD.pdf.
Once you know the number of cubic centimeters it takes to hold the powder charge you wish to dispense, you can choose the proper disks. You choose the disk combination that is closest in size to the value of the formula without going over. If you have a scale check the weight of the powder charge dispensed by the disk combination.