Techniques for improving implementation of a downhole tool string to be deployed in a borehole formed in a sub-surface formation. In some embodiments, a design device determines a model that describes expected relationship between properties of the downhole tool string, the borehole, the sub-surface formation, and mud cake expected to be formed in the borehole; determines calibration locations along the borehole based on properties of the borehole; determines candidate spacer configurations based on contact force expected to occur at contact points between the downhole tool string and the mud cake when deployed with each of the candidate spacer configuration via the model; and determines a final spacer configuration to be used to attach one or more spacers along the downhole tool string based on expected head tension to move the downhole tool string when deployed in the borehole with each of the candidate spacer configurations via the model.