how to sum period on a liability account

I have a requirement to show aggregated amounts of a liability account, for a given FY.

LIABILITY1  for AUG should show the sum of first 8 months liability, i know it is a behavior of expense account on a liability account. For clarity sake,

LIABILITY1 will be a stat account created using actual LIABILITY account using MDX formula.

1) Tried defining LIABILITY1 as EXP account where in the MDX has it as "= LIABILITY"   It didn't do the trick to mimic exp aggregation behavior for year total.

2) Custom measure is an option but i would like to look other options as custom measures are performance hogs.

Please note , excel formulas as workbook solution are not accepted here.