Remember .... Those figures are just a rule of thumb. The amount of ventilation you really need may be a lot more. Here in Texas it isn't uncommon for one whole wall to be left open for ventilation. We have to deal with heat and little cold. My ventilation needs are going to be much different...
As suggested above .... this is how I worked the back of my shed for more ventilation. Of course the soffit board was replaced and the whole area covered in 1/2 HC with screws and washers. Works good for me.
Edited : Correction - the soffit board was not replaced, it was left open and covered...