The problem is that the two sides must grow at different rates with the same number of pleats so that the folds MUST be different sizes.

It is much easier to do it twice, the first time glue it up on heavy paper completely and make sure the design works correctly. This will be a very difficult bellows to build because of the difference is sizes.

Once the paper one works, cut it down one corner to see the size and shape you need the leather to be.

Good luck.