I live in Belgium and use the A3 format which is 11.7 x 16.5 inches

No, that's not what I mean. I have difficulties explaining it in English.
The Hahnemuhle content paper is already pre-creased (bended? folded? ...) and has holes in it for using it in a screw post binder.
This means that there is an inch or so that you don't use for printing.

I can't find out how to add an image to this post so I can't show what I mean.

I will have 1 photo on each page, visibly centred, which means that I will be off center because It needs to  be centered between the position of the creasing and the edge of the paper. And then of course it will be a different position on the other side of the paper.

I will for the first time print double sided on Hahnemuhle Photo rag duo content paper.
It is the A3 (11 x 17inch) landscape format.
I want to avoid wasting to much paper by experimenting before I find the right layout and approach for the double sided printing.
Is there someone who has experience with this and can give me some tips, maybe a template?

Thanks

Thanks for your reply
What do you mean with "add a mat"?

I try to find out the settings in Qimage ((V2020) for printing a panorama on 17 inch roll paper with the Epson P800.
I want to leave a margin of approx 2 cm on top and bottom. This means that W should be +- 380 mm and L = ?
The size of the photo is 20691 x 4982 pixels

I don't find the right settings. I also read some older related topics on this forum but I still have no clue.

Can someone please give some advice?

Thanks