To be legal you need a full current chart, folded any way you like. Does anybody know if it has to be half-mill? I thought 1/4 mill was OK too.
But the chart you actually use can be anything. This is what I do:
Buy a second chart, and cut it into a big square with your "home airfield" in the centre. The aim is to fold this into 16 little squares so choose the size appropriately.
Fold it vertically down the middle, crease it.
Fold it horizontally across the middle, crease it.
The next bit is a little fiddly, but makes folding easier later.
Make two more vertical folds, to divide the big square into four vertical strips.
DO NOT fold at the exact quarter points, but about 2cm or so away from the centre, ie toward the edge. This allows the outside strips to easily fold under the central ones.
Do the same thing horizontally.
You can now fold this into a variety of 2x2, 2x3, 3x2 configurations, secured with bulldog clips.
You can also have extra charts with the base airfield at the extreme left, extreme top, etc, depending on the airspace around you and your typical routes. You can try 36 square versions too, always making the outer strips smaller than the inner ones.
The benefit of cutting is it makes folding easier. The more layers you have, the harder it is to fold. I believe there is a rule in topology that you can't fold anything more than 7 times. Try it with a bath towel! (Actually, it probably isn't topology...)
The obvious drawback of cutting is reduced coverage, but in practice it is hard to re-fold a big chart anyway. Or even suicidal (eg R22 solo...) In that situation I would prefer multiple overlapping charts.
I have seen big charts folded so that they "concertina", but I think this only works well if you are going North/South or East/West. I am more than happy to be enlightened about that!