I am using below formula for lens thickness calculation and then comparing my results with OptiCampus calculator:

Thickness = MinimumThickness + ( ( ((ED+A+DBL-PD)/2)^2 x |Sph+(Cyl x sin(Axis)^2)| ) / (2000*(Index - 1)]) )

My results compare close enough except when I have a +ve Sph power with -ve Cyl
For example +2.00 -2.00 x 90
Gives 2.7mm CT and 2.7mm ET@180 on OptiCampus

WHILE above formula gives 2.0mm CT and 2.0mm ET@180