Stock Leger Retail method - How can you adjust the cum_markon_pct

For stock ledger, retail method, how can the CUM markup be "corrected" or "reset" if you feel that it's incorrect.  We have one that is too low in a very active dept/class/subclass/warehouse location.  Purchases and transfers are at a high markup.  It looks like whenever the ending stock gets too low, it books a negative gross margin dollars to keep the CUM from going negative.  If the ending stock is higher the next week, then it books an offsetting positive gross margin dollar amount.  What can we do to stop this?  In one particular d/c/s, the negative gross margin was