But I find the better rule of thumb, for me at least, is to think of a normal lens as the length of the long side of the film. Thus for 4x5 I think of 127mm (or 135, depending on which camera body - I don't have all lenses for all bodies!) as normal, and for 135 I think of 35mm as normal. So my "standard" is a bit wider than the common usage.
I don't shoot wild sizes like 12x20.