@luxire, I assume that the sale shoes cannot be made as per standard specs but on custom lasts?
Also, may new bespoke shoe orders (those with existing lasts) be added for a special? If so, please tomorrow, I have to sleep :).
If the problem is your shirts falling out, I'd increase them the same amount as not to alter the balance, but not by as much as 5 cm. I'd say 'increase front and back rise near the waistband by 3 cm'. You'll likely also need to let out the waistband as well. You can also consider rubber grippers and longer shirts.