To Work Late or To Work Really Late?

The Consulate's official working hours are 8:00am to 5:00pm. These hours are an hour early than typical Sao Paulo working hours due to the traffic. Leaving my house at 7:30am (yes, this is an extremely painful part of my day), it takes me twenty minutes to get to work; while leaving at 8:00 or 8:30 would require forty or so minutes. The math is clear!

The same is true for the trip home. If I manage to leave at 5:00pm, which is never, it will take me 35 or so minutes to get home. If I leave at 6:00 or 6:15pm, I will take me over an hour to make the journey. If I leave at 7:30 or 8:00pm, the transit time has returned to 35 minutes. The math is no longer so clear ... do I work a little bit of overtime and take over an hour to get home (the consequences being transit rage, but the possibility of completing other activities), or do I work a lot of overtime and get home quickly (the consequences being transit zen, but limited completion of other activities)?

There must be some kind of formula to figure this out! If x = transit time; y = productivity at work; z = loss or gain of after-work activities; and a = general happines, then ....