The flatjack deformability testing described in ASTM C1197 has been used effectively for over 25 years to evaluate compressive stiffness and strength of in situ masonry assemblies. The test method is used most frequently at historic masonry structures where the mechanical properties of the masonry are unknown. The test involves cutting vertically aligned horizontal slots into the masonry assembly, inserting hydraulic bladders (flatjacks), and inflating these bladders while monitoring displacement of the masonry. This procedure produces a stress-strain curve that can be used to determine the assembly stiffness and often its strength. In the current standard, the pressure applied by the flatjack is adjusted by an area reduction factor, K_a. This factor effectively reduces the calculated pressure applied by the flatjack by the ratio of the flatjack surface area to the area of the slot. This reduction factor assumes that the pressure from the flatjack will be spread evenly over the slot area, both through the wall thickness and along the slot length. If the area reduction factor is analyzed for an extreme condition, cutting a slot two times the length of the flatjack will result in calculated compressive stresses in the test area of one-half the magnitude of the stresses under the same pressure with a tightly fitted slot. Clearly, this would not be the case. The length of the slot can reasonably be expected to have some impact on stress levels near the edges of the test specimen, but the effects of longer slots on stress levels near the center of the test area are less obvious. This paper describes the numerical results of the finite element model analysis associated with parametric studies of slot length and displacement measurement locations for ASTM C1197 testing. Recommendations for modifications to the K_a factor are also provided.