"Maximum productivity temperature" is whatever doesn't have me shivering or sweating. In the winter (PNW, west of Mt. Tahoma) we keep our house at 59-61F, and in the summer we let it get up to 76-78F before cooling it down (will use fans to cool us down, though, if it's not too humid).
If it's just me in an outbuilding I'll put on a hat and coat and be okay in 41F (the lowest I've noted while also being comfortable enough typing). "Optimal" in that case includes the cost to heat the space, which I'd rather not do if I don't have to.