Updating multiple element input values

Summary:

I have a Pension plan where employee can elect the pension %. I have enabled this by using a Coverage which has been defined as a flat amount enter at enrolment.

I have two rates, Pension amount and Pension Percentage and I would like both of these rates to update the same element which has two input values; Employee Charge and Pension Percentage

I have tried using an Extra Input formula but am not sure how to get the coverage amount and the rate amount on the Pension option 'Select your Pension Contribution % (5 to 80%)'

l_cvg_amt_char = ben_fn_get_char_value('BEN_PRTT_ENRT_RSLT'