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Potential sources of error in dosage planning in strabismus surgery are (a) prismatic side-effects of spectacle lenses when measuring the preoperative angle with the alternating prism cover test and (b) a potential influence of eye ball axial length on dose response. As both errors take effect in opposite directions, many strabismus surgeons set aside their consideration. This study investigates whether considering both factors for dosage planning yields better operative results. In this prospective, randomised, double-blind, interventional pilot study, we included patients scheduled for purely horizontal strabismus surgery and determined each patient’s surgical dose (total amount of recession/plication) either with (study group) or without (control) consideration of the two factors. The deviation of the resulting angle from the target angle 3 months postoperatively was the primary endpoint. One hundred one patients were included, 51 of which in the intervention group and 50 in the control group. The primary endpoint showed a median deviation from the target of 3.0° in the intervention group and 4.8° in the control group. We observed a group difference of 1.8° in favour of the intervention group (p = 0.053). Subgroup analysis showed a difference between groups of 2.2° for esotropic patients and of 5.1° for patients with hyperopia > + 2 D. Taking prismatic side-effects of spectacle lenses and eye ball length into account when calculating strabismus surgery doses showed a trend towards more accurate results. Esotropic patients and patients with hyperopia > + 2 D seemed to benefit most. International Clinical Trials Registry Platform: DRKS00011121