In custom software development projects, software acquirer always participates in software development processes acting as collaborator, reviewer or supervisor. The acquirer participation has a significant influence on project performance. In practice, the plans of acquirer participation are often made by project managers according to their personal experience or intuition, and the quality of the plan is difficult to guarantee. Moreover, because of lacking objective evaluation criterion for the plan, it's hard for the acquirer and the supplier to reach an agreement or compromise when they have different opinions. To solve this problem, this paper collects historical data of 25 software projects and establishes a quantitative model of relationship between the acquirer participation and project performance by leveraging a variable-selection-based regression analysis approach. The quantitative model provides guidance for making acquirer participation plans. Furthermore, a review of 9 experienced project managers was conducted to evaluate the model's validity.