Your PE (Performance Equation) depends on what,when and where you want to calculate your formula

If you only want to display the result at the end of the week (Monday 7am) then you can use this:
TimeLE('sinusoid',('mon+7h')+'-7d',('mon+7h'),20)/(('mon+7h')-(('mon+7h')+'-7d'))*100

TAGNAME:sinusoid=>Replace with your tag
START TIME:('mon+7h')+'-7d'=>This will take the current week start and subtract 7 days
END TIME:('mon+7h')=>Current Monday + 7 hours
You can replace the 20 with any figure you want.

If you want to display this result as "running" calculation you will need to be more creative with your end time, consider the Totalization subsystem and if you have PI-ACE you can use that.