This formula is the sagittal approximation formula which uses the power to approximate the thickness. You will also need to add the minimum thickness to get a better approximation.
To get a more accurate result you will need to find the refractive front and back powers using the refractive power formula.(note to get the power needed on the back surface you will use the back vertex power formula)
Then find the radius for each surface, then find the sagittal depth for each surface
Plus lenses, you find the center thickness by:
Ct = f1_sag - f2_sag + min. Thickness
ET = Min Thickness
For Minus lenses:
CT = Min Thickness
ET = F2_sag - F1_sag + min thickness.
I have worked the steps out for you on my website
http://brent-mccardle.org
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