The following data pertains to the month of October for ElmCo. when production was budgeted to be 5,000 units of P90. P90 has standard costs per unit of: 3 lbs. of Direct Materials at a cost of $7.00 per lb.; 0.20 hours of Direct Labor at $18.00 per hour; and Variable Overhead assigned on the basis of 0.05 machine hours at a rate of $50 per machine hour. Actual production of P90 for October was 4,600 units. In October the production of P90 totaled 4,600 units, using 324 machine hours costing a total of $17,066. Determine the variable overhead efficiency variance. (Negative numbers indicate a favorable variance.)