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I think it would be great if you could just, you know, open the Cura Manual or read a bit about print prep.

The total shell thickness is linked to 2 parameters (in FDM printing) : the diameter of the printer nozzle and the number of shell-paths. Say you have a 0,4mm diam nozzle, you print at 0,45mm width and you decide to have 3 shells : total 1,35mm. It is the max external paths thickness on any horizontal slice (layer).

The infill % is basically for any slice of your part the % of surface that is filled vs the total available surface.

If at that point you're wondering why I said slice in the above sentence, please follow advice number one and find some data on slicers.

The software also allows you to decide how many layers are completely filled at the bottom and top. This is how you get to decide the shells on the vertical axis.

Now, as you can see, you set a discreete number, there is no scaling. This is not part of the design either, this is set in the print prep software, generally called slicer.

If I read you correctly, you intend to do a 1,5mm thick support ? I'm sorry but seriously, I don't need to open a Finite Element model to tell you that it won't sustain the weight of a 1kg Surface, unless you just want to use it as a shim ? Even an aluminium sheet would probably bend noticeably.

I may err on the side of over-engineering but give considerations to the dimensions I gave you. Unless of course, you just want to raise the tablet, not actually support it ?