During WW2, why do they say 60 degrees sloped armor is the best angle for sloped armor on tanks such as T34?

I know the good properties sloped armor, deflection plus creating on a flat trajectory a greater depth to for the same thickness of metal. Is 60 degrees the ideal angle. I thought an even more shallow angle such as 30 or 45 degrees would be even better (Like the upper frontal sloped hull of the M1A1 Tank). Why in WW2 did the T34 Tank go for 60 degrees, was it the best deflection angle? Cheers John.