You might want to add a column for OT earned M-F and one for OT earned Sa-Su so that you can distinguish between the two.
So you'd have the following totals columns:
RT: =IF(SUM("mon-fri") > 40,40,SUM("mon-fri"))
OT1: =IF(SUM("mon-fri") > 40,SUM("mon-fri")-40,0)
OT2: =SUM("sat-sun")
OT: =OT1 + OT2
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