Max Merch is the correct way. The information is on shelf level and will impact only the planogram for the specific store you are working on.
Please be aware that in case you copy the shelf, the Max Merch dimension will also be copied.
We recommend not to set up a Max High for a product. This information is on product level and it will change the product dimensions in all planograms from this AssG.
We strongly advise to clear this field for all articles, setting the Max High to zero.
Please be aware that in case you copy a planogram, the Max High will be copied as well.