Any person electing to come within this chapter may elect not to claim full credit for his prior service, but may elect to receive credit for only a fractional part of such prior service. In such event, he shall receive a refund or make deposit based on the amount to his credit in the other pension system as against the fractional part of the prior service claimed. The amount necessary for such credit shall be determined on a 2 percent basis applied to the aggregate salary or wages paid during the period of prior service claimed.