Lookup on Line item subset "Parent"




I have a collect() module dimensioned by a line item subset and time and another by account and time, the account module is referencing the collect module using a lookup based on a mapping. 


The mapping dropdown allows me to select any member in the LISS and the formula works if I pick the first member i.e. Rent, however selecting a LISS member such as total turnover returns 0 as total turnover is a summary of rent and other line items in the original source module. 


Does anyone know a workaround for this to enable a lookup against any member in the line item subset?