I am referring to modern, small incision (about 4-5 mm), no-stitch surgery where the incision is place at the lateral 3 or 9 o'clock meridian on the cornea. Such an incision may reduce "against the rule' astigmatism about 0.25-0.50 diopter or increase "with the rule astigmatism" by a like amount. There is no formula to calculate visual acuity without glasses in an otherwise normal eye with regular corneal astigmatism.