Achieving robotic touch that is accepted and trusted by human users requires a clear understanding of touch parameters and how they influence perceptions of safety and comfort. We explore such parameters through a participatory experiment and generate design guidelines for robotic touch, involving 20 participants in designing, evaluating, and iterating robotic parameters for instrumental touch using a robotic arm equipped with a robotic hand. Through our user study, we find that parameters such as robot's speed, force, and motion, contact surface material, and the robot hand pose impact users' perception of safety, comfort, intuitiveness, efficiency, effectiveness, trust, and sense of agency. We also find that the choice of robotic parameters is task-dependent and body-part dependent, and that these parameters inter-correlate with each other. Based on these findings, we provide design guidelines for generating user-friendly robotic touch interactions in instrumental touch contexts.