Is there any way to automate the following equation so that every 7 days the
final part of the equation (BUS!J39) automatically changes to a cell
reference 6 colums up from the one previously referenced e.g(BUS!P39 and so
on etc)?

=SUMPRODUCT((MOD(COLUMN(BUS!J39:ER39),6)=MOD(COLUM N(BUS!J39),6))*(BUS!J39:ER39))-BUS!J39