Assistance with using data to give a number then take a percentage off that number

Hi team,

I've got myself stuck. The basic is I want to calculate a column have an answer then in the same cell get a % from the returned amount.

um({Payroll Details.Notional Weekly Earnings}*85%)RunningSum(*)79.959%) This is what I have, I know it's so very wrong please help.

PS the first section Minus the SUM part works ({Payroll Details.Notional Weekly Earnings}*85%), it's the end that doesn't