How to calculate corridor %

The following information relates to the pension plan for the employees of Turner Co.: 12/31/18 12/31/19 12/31/20 Accum. benefit obligation $5,280,000 $5,200,000 $5,520,000 Projected benefit obligation (PBO) 5,100,000 6,880,000 6,788,000 Fair value of plan assets 5,580,000 8,004,000 5,976,000 AOCI net (gain) or loss -0- (960,000) (864,000) Settlement rate (for year) 11% 9% Expected rate of return (for year) 7% 8% Turner estimates that the average remaining service life is 15 years. Turner's contribution was $756,000 in 2018 and benefits paid were $564,000. The corridor for 2020 and its % is ? i know you multiply the highest of PBO or FV with a % i just dont understand from where do you get that % rate show please explain and show how u get that %