In determining the amount earnable by a member in any payroll period, the board of trustees may consider the rate of annual compensation payable to such member on the first day of the payroll period as continuing throughout such payroll period and it may omit deduction from compensation for any period less than a full payroll period if the policeman was not a member on the first day of the payroll period and to facilitate the making of the deductions, it may modify the deduction required of any member by such amount as shall not exceed one-tenth of one percent of the compensation upon the basis of which such deduction was made.