Now if you wanted a STFTBEN you would define a suitable calc pid (if you don't already have one)...
for example, any one of these (take your pick):
1+{GM.SHRTFT1}/100
1+{GM.SHFTFT2}/100
1+({GM.SHRTFT1}+{GM.SHRTFT2})/200
any one of those ^ give you a BEN correction multiplier... you would make a map with this pid on the Data tab.