Not sure if you figured this out yet, but I took a shot at it. My function might be a little bit(or extremely) excessive, but I need some practice so what the heck. I didn't get around to adding in whether it discerns between weekdays/weekends, but it might get something going.
It assumes an 8:00AM-6:00PM Work day.
Code:
Expr1: IIf(([ProcessTime]+Hour([StartDate]))<=18,DateAdd("h",[ProcessTime],[StartDate]),DateAdd("h",8+(((([ProcessTime]-(18-Hour([StartDate])))/10)-Int(([ProcessTime]-(18-Hour([StartDate])))/10))*10),DateAdd("d",1+(Int(([ProcessTime]-(18-Hour([StartDate])))/10)),DateAdd("h",-1*(DatePart("h",[StartDate])),[StartDate]))))