Difference of 2 calculated fields in Pivot in Spotfire -

i have raw data on account level current balance, total balance , sales amount formula reads:
total dso : total balance * 92 / 3monthbilling amount
best dso : current balance * 92 / 3monthbilling amount

i wanted grouping on month level overall dso, best dso used formula:
total dso : total balance * 92 / sales amount on (month)
best dso : current balance * 92 / sales amount on (month)

the above 2 works want 3rd field called remaining dso = total dso-best dso

so in spotfire, when insert calculated column pivot saying remaining = total dso - best dso don't correct value.

then tried remaining = total dso - best dso on (month) , remaining = (total balance-current balance)*92 / sales on (month) both produced incorrect value.

simply, want difference of 2 fields (but done on pivot level excel , not raw data level)