This Merida manual on page 130 says 5–7Nm or 6–8Nm, depending on which bolt size the clamping mechanism has. I’m not 100% sure the manual applies to your seat post but I assume going up to 7Nm should be safe.
You could try carbon installation compound or a retaining compound (e.g. Loctite 603). I’ve also used liquid climbing chalk (magnesium carbonate) when I had nothing else at hand. Use a waterproof marker to mark the position of the saddle rails so you know if it got loose.
The best approach is to have a manufacturer specification. It’s always annoying when there is no specification or it’s unclear for which product the specification actually is. I really don’t understand why they can’t just print it on the part and/or provide a proper manual for each part.
The good news in case of the saddle clamp is that it’s not safety critical.