Markup limit on roofing materials sold as auto-entrepreneur?

I have a roofing business set up in France as an auto-entrepreneur and as per your reply to my previous question I am paying my business cotisations at the lower rate on the materials I sell to my clients and at the higher rate on my labour.

As I have to pay cotisations on the sale of materials I have been marking up the materials on the devis and facture so that it covers the cotisations I have to pay, or otherwise I would make a loss on the supply of materials and would have to increase my labour charge to cover this loss and pay cotisations on this at the higher rate.

What I would like you to confirm is whether there is some limit as to how much my business can mark up the sale of materials? Clearly it would benefit me to charge more for materials and lower the labour charge (with the same total charge to the client) as I would pay the lower rate on more of my turnover.

Also my total labour can not exceed 32,900 euros but the combined limit for labour and sales of materials is 82,200 euros so I could make more profit on the materials and charge the labour at a lower price which means that I could actually do more work in the year and still remain an auto-entrepreneur.