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Select the initial diagnostic lens dependent upon the amount of corneal cylinder:

Corneal Cylinder Base Curve
0.00 to 0.75D Fit on flat K
0.87 to 1.50 Fit 0.25 steeper than flat K
1.62 to 2.25  Fit 0.50 steeper than flat K
2.37 to 3.00 Fit 0.75 steeper than flat K

The diagnostic lens should position centrally without impinging on the limbal conjunctiva and exhibit some movement, though minimal. The fluorescein pattern should be relatively uniform with mild central pooling and with no seal off or touch. After the proper base curve corneal relationship has been achieved, perform a spherical over-refraction to best acuity. Comfort Zone-5 will neutralize as much as 2.50 D cylinder.

A 10.0mm diameter is traditionally used, however 9.5mm and 10.5mm diameters are available.