<unk>  is a mathematical formula used to estimate astigmatism based on keratometry readings. The estimate is useful for high cylinder measures, generally over -2.00 diopters. The estimate is found by multiplying the difference in power between the two meridians by 1.25 and factoring in the average lenticular astigmatism, which is -0.50 x 090.
Javal's rule
Clark's rule
Goodsall's rule
Eichler's rule
Köllner's rule
Haber's rule
Murphy's sign
Westgard rules