Maybe
Code:
= IIF(LunchTaken = True,Sum((Round(([Finish]-[Start])*24,2))-0.5)-30,Sum((Round(([Finish]-[Start])*24,2))-0.5))
I might suggest to pass the value of Sum((Round(([Finish]-[Start])*24,2))-0.5) to another textbox or variable so that this could be
Code:
txtSum = Sum((Round(([Finish]-[Start])*24,2))-0.5)
and then in other textbox
Code:
= IIF(LunchTaken = True,txtSum-30, txtSum)
However, I might make the textbox equal to a function that makes this calculation as what I've posted doesn't cover situations where your value(s) could be Null.